[MCQs] Carbohydrate Metabolism Quiz (32 test)

Carbohydrate Metabolism Quiz (32 test)

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Question 1

Diarrhea from infection or malnutrition is the world’s most prevalent killer of children. A 2-month-old Caucasian girl develops chronic diarrhea and liver inflammation in early infancy when the mother begins using formula that includes corn syrup. Evaluation of the child demonstrates sensitivity to fructose in the diet. Which of the following glycosides contains fructose and therefore should be avoided when feeding or treating this infant?

A	Sucrose
B	Ouabain
C	Lactose
D	Maltose
E	Streptomycin

Question 1 Explanation:

Glycosides are formed by condensation of the aldehyde or ketone group of a carbohydrate with a hydroxyl group of another compound. Other linked groups (aglycones) include steroids with hydroxyl groups (eg, cardiac glycosides such as digitalis or ouabain) or other chemicals (eg, antibiotics such as streptomycin). Sucrose (α-D-glucose-β-1 → 2-D-fructose), maltose (α-D-glucose-α-1 → 4-D-glucose), and lactose (α-Dgalactose-β-1 → 4-D-glucose) are important disaccharides. Fructose is among several carbohydrate groups known as ketoses because it possesses a ketone group. The ketone group is at carbon 2 in fructose, and its alcohol group at carbon 1 (also at carbon 6) allows ketal formation to produce pyranose and furanose rings as with glucose. Most of the fructose found in the diet of North Americans is derived from the disaccharide sucrose (common table sugar). Sucrose is cleaved into equimolar amounts of glucose and fructose in the small intestine by the action of the pancreatic enzyme sucrase. Deficiency of sucrase can also cause chronic diarrhea. Hereditary fructose intolerance (MIM*229600) is caused by deficiency of the liver enzyme aldolase B, which hydrolyzes fructose 1-phosphate.

Question 2

Which of the following carbohydrates would be most abundant in the diet of strict vegetarians?

A	Amylose
B	Lactose
C	Cellulose
D	Maltose
E	Glycogen

Question 2 Explanation:

Cellulose, the most abundant compound known, is the structural fiber of plants and bacterial walls. It is a polysaccharide, consisting of chains of glucose residues linked by β1 → 4 bonds. Since humans do not have intestinal hydrolases that attack β1 → 4 linkages, cellulose cannot be digested but forms an important source of “bulk” in the diet. Lactose is a disaccharide of glucose and galactose found in milk. Amylose is an unbranched polymer of glucose residues in β1 → 4 linkages. Glycogen is a branched polymer of glucose with both β1 → 4 and β1 → 6 linkages. Maltose is a disaccharide of glucose, which is usually the breakdown product of amylose.

Question 3

Alcohol abuse affects over 13% of adults in the United States, costing more health care dollars ($184 billion annually) than cancer ($107 billion) or obesity ($100 billion) in some studies. In 2001, 47% of those between ages 12 and 20 years reported drinking, 30% of these to binge drinking in the last month. Chronic alcoholics require more ethanol than do nondrinkers to become intoxicated because of a higher level of a specific enzyme. However, independent of specific enzyme levels, the availability of what other substance is rate-limiting in the clearance of ethanol?

A	NADH
B	NAD+
C	FADH
D	FAD+
E	NADPH

Question 3 Explanation:

In humans, ethanol is cleared from the body by oxidation catalyzed by two NAD+-linked enzymes: alcohol dehydrogenase and acetaldehyde dehydrogenase (eliminating answers a, c-e). These enzymes act mainly in the liver to convert alcohol to acetaldehyde and acetate, respectively. In chronic alcoholics, alcohol dehydrogenase may be elevated somewhat. The NADH level is significantly increased in the liver during oxidation of alcohol, owing to the consumption of NAD+. This leads to a swamping of the normal means of regenerating NAD+. Thus, NAD+ becomes the ratelimiting factor in oxidation of excess alcohol.

Question 4

In lung diseases such as emphysema or chronic bronchitis, there is chronic hypoxia that is particularly obvious in vascular tissues such as the lips or nail beds (cyanosis). Certain genetic diseases such as α1-antitrypsin deficiency (MIM*107400) predispose to emphysema, as do environmental exposures such as cigarette smoking or asbestos. Poorly perfused areas exposed to chronic hypoxia have decreased metabolic energy for tissue maintenance and repair. Which of the following is an important reason for this?

A	Increased hexokinase activity owing to increased oxidative phosphorylation
B	Increased ethanol formation from pyruvate on changing from anaerobic to aerobic metabolism
C	Increased glucose utilization via the pentose phosphate pathway on changing from anaerobic to aerobic metabolism
D	Decreased ATP generation and increased glucose utilization on changing from aerobic to anaerobic metabolism
E	Decreased respiratory quotient on changing from carbohydrate to fat as the major metabolic fuel

Question 4 Explanation:

The exposure of tissues to chronic hypoxia makes them rely more on anaerobic metabolism for the generation of energy as ATP and other high-energy phosphates. Most tissues except for red blood cells can metabolize glucose under anaerobic or aerobic conditions (red blood cells do not have mitochondria for electron transport and must rely on other tissues to generate glucose back from lactate). In most tissues, a switch from aerobic to anaerobic metabolism greatly increases glucose utilization and decreases energy production. (Increased glucose utilization under anaerobic conditions in bacteria is known as the Pasteur effect after its discoverer.) Under aerobic conditions, the cell can produce a net gain, in moles of ATP formed per mole of glucose utilized that can be as high as 18 times that produced under anaerobic conditions. Thus, the cell generates more energy and requires less glucose under aerobic conditions. Such increased ATP concentrations, together with the release of citrate from the citric acid cycle under aerobic conditions, allosterically inhibit the key regulatory enzyme of the glycolytic pathway, phosphofructokinase. Decreased phosphofructokinase activity decreases metabolism of glucose by glycolysis.

Question 5

An 18-year-old Korean college student attends a fraternity party and is embarrassed because he becomes flushed and sick to his stomach after his first drink of alcohol. He learns that this reaction is due to genetic variation in some Asians and Native Americans that affects metabolism of a metabolite of alcohol. Which of the following is the variably degraded metabolite?

A	Methanol
B	Acetone
C	Acetaldehyde
D	Hydrogen peroxide
E	Glycerol

Question 5 Explanation:

The principal pathway for hepatic metabolism of ethanol is thought to be oxidation to acetaldehyde in the cytoplasm by alcohol dehydrogenase. Acetaldehyde is then oxidized, chiefly by acetaldehyde dehydrogenase within the mitochondrion, to yield acetate. Acetone, methanol, hydrogen peroxide, and glycerol do not appear in this biodegradation pathway, eliminating answers a-b, d-e. The genetic variations of acetaldehyde dehydrogenase have few phenotypic effects aside from sensitivity to alcoholic beverages and are extremely common in the affected populations. These characteristics qualify acetaldehyde dehydrogenase variation as an example of enzyme polymorphism. Alcohol sensitivity is less of a factor in alcohol overdose or poisoning, where large intake overwhelms the liver’s capacity to metabolize about ounce per hour. Symptoms of coma and impending respiratory arrest mandate emergency room treatment to support respiration, avoid aspiration, reverse hypoglycemia (with IV dextrose), and restore depleted thiamine.

Question 6

A 6-month-old Caucasian girl is hospitalized for evaluation of short stature, enlarged liver, and intermittent lethargy/irritability that, on a recent emergency room visit, was accompanied by low blood glucose (hypoglycemia). In the same ward is a 6-year-old African American boy who is being evaluated for severe muscle cramping that raised suspicions for sickle cell disease. The family history is unremarkable for these children; each has a normal sibling and normal parents. Both children are given diagnoses of glycogen storage disease with the infant girl having type I (MIM*232200) affecting liver and the older boy type V (McArdle disease, MIM*232600) affecting mainly muscle. Which of the following conversions explains the difference in these presentations?

A	Glycogen to lactate in liver
B	Glycogen and lactate to glucose in liver
C	Glycogen to glucose in muscle
D	Glycogen to alanine in muscle
E	Glycogen to glucose 6-phosphate in liver

Question 6 Explanation:

Glycolysis in muscle produces lactate, which must be converted to glucose by liver or kidney via the Cori cycle (incorrect answers a, c, and e). Defects in liver glycogen metabolism therefore impair glucose 6-phosphate production or gluconeogenesis (alanine is also a substrate for gluconeogenesis—incorrect answer d) with resulting hypoglycemia and liver glycogen storage with or without toxicity (cirrhosis). Defects in muscle glycogen metabolism impair contraction (cramps, fatigue) with decreased serum lactate production during exercise and muscle glycogen accumulation (progressive weakness and atrophy). Glucose 1-phosphate is the first intermediate in the conversion of glycogen to glucose. The enzyme glycogen phosphorylase catalyzes this first step. The second intermediate, glucose 6-phosphate, is subsequently converted to glucose by the enzyme glucose-6-phosphatase. This enzyme is found only in the liver and kidney; thus, these are the only tissues able to break down glycogen for use by other tissues. In tissues such as muscle, glycogen can be broken down to glucose 6-phosphate but can only be used in the cell in which it was produced.

Question 7

A 3-day-old boy from an Iraqi refugee family exhibits severe tissue swelling (edema), rapid heart rate (tachycardia), enlargement of liver and spleen (hepatosplenomegaly), and jaundice of the eyes and skin. The couple had two prior female infants who are alive and well, and the wife relates that she lost a brother in infancy with severe hemolysis induced after a viral infection. Blood tests show hemoglobin of 5 g/dL (normal neonate mean 18.5) with reticulocyte count of 15% (normal 1.8-4.6). Exchange transfusion is attempted but the child suffers cardiac arrest. Postmortem assay of red cell enzymes confirms the suspected diagnosis of glucose-6-phosphate dehydrogenase deficiency (MIM*305900), implying defective synthesis of which of the following compounds?

A	Deoxyribose and NADP
B	Glucose and lactate
C	Lactose and NADPH
D	Ribose and NADPH
E	Sucrose and NAD

Question 7 Explanation:

Glucose-6-phosphate dehydrogenase (G6PD) is the first enzyme of the pentose phosphate pathway, a pathway that metabolizes glucose to produce ribose and NADPH (eliminating answers a-c, e). Its deficiency (MIM*305900) is the most common enzymopathy, affecting 400 million people worldwide. It contrasts with glycolysis in its use of NADP rather than NAD for oxidation, its production of carbon dioxide, its production of pentoses (ribose, ribulose, xylulose), and its production of the high-energy compound PRPP (5-phosphoribosyl-1-pyrophosphate) rather than ATP. Production of NADPH by the pentose phosphate pathway is crucial for reduction of glutathione, which in turn removes hydrogen peroxide via glutathione peroxidase. Erythrocytes are particularly susceptible to hydrogen peroxide accumulation, which oxidizes red cell membranes and produces hemolysis. Stresses such as newborn adjustment, infection, or certain drugs can increase red cell hemolysis in G6PD-deficient individuals, leading to severe anemia, jaundice, plugging of renal tubules with released hemoglobin, renal failure, heart failure, and death. Since the locus encoding G6PD is on the X chromosome, the deficiency exhibits X-linked recessive inheritance with severe affliction in males and transmission through asymptomatic female carriers. Ribose 5-phosphate produced by the pentose phosphate pathway is an important precursor for ribonucleotide synthesis, but alternative routes from fructose 6-phosphate allow ribose synthesis in tissues without the complete cohort of pentose phosphate enzymes or with G6PD deficiency. The complete pentose phosphate pathway is active in liver, adipose tissue, adrenal cortex,thyroid, erythrocytes, testis, and lactating mammary gland. Skeletal muscle has only low levels of some of the enzymes of the pathway but is still able to synthesize ribose through fructose 6-phosphate.

Question 8

Which of the following best explains why fructose was formerly recommended for patients with diabetes mellitus?

A	Fructose is a better substrate for hexokinase.
B	Fructose stimulates residual insulin release.
C	Fructose has a specific kinase in liver that allows bypass of phosphofructokinase.
D	Fructose is phosphorylated and cleaved to triose phosphates, which cannot be used for gluconeogenesis.
E	Hexokinase phosphorylates fructose in extrahepatic tissues, and its activity will not be affected by high glucose concentrations in diabetes.

Question 8 Explanation:

A special pathway for fructose metabolism (a specific fructokinase plus aldolase B and triokinase) is present in liver, kidney, and small intestine (incorrect answers a, b, d, and e). Foods high in sucrose (glucose-fructose) such as syrups, beverages, or diabetic substitutes yield high concentrations of fructose in the portal vein. Fructose is catabolized more rapidly than glucose by its specific fructokinase, bypassing hexokinase that is regulated by fasting and insulin. While providing a fuel for glycolysis, fructose also increases fatty acid, VLDL, and cholesterol-LDL production that are also side effects of diabetes mellitus due to the necessary shift to fat oxidation when intracellular glucose is less available. Recent data also suggest that fructose in soft drinks and other foods promotes insulin resistance and type II diabetes. For these reasons, the American Diabetic Association (www.diabetes.org) now suggests avoidance of fructose with the exception of naturally occurring fructose in fruits Fructose metabolism begins when fructokinase catalyzes the phosphorylation of fructose to fructose 1-phosphate, which is then split to D-glyceraldehyde and dihydroxyacetone by aldolase B. Triokinase converts D-glyceraldehyde to glyceraldehyde 3-phosphate, which can be metabolized further by glycolysis or be condensed with dihydroxyacetone phosphate by adolase to form fructose 1,6-diphosphate, glucose 6-phosphate, and glucose through gluconeogenesis.

Question 9

A 2-month-old Caucasian girl is evaluated for hypoglycemia and lactic acidosis and noted to have an enlarged liver. Biopsy reveals stored glycogen, and a glycogen storage disease is suspected. Assay of usual glycogen enzyme deficiencies in the liver specimen is normal, so the metabolic consultant recommends assay of rarer enzyme deficiencies that influence glycogen metabolism. These enzymes would most likely include which of the following?

A	Hexokinase
B	cAMP-dependent protein kinase
C	Glucose-6-phosphate dehydrogenase
D	Phosphofructokinase
E	Fructose-1,6-diphosphatase

Question 9 Explanation:

Glycogen synthesis and breakdown (glycogenolysis) are accomplished by separate pathways rather than reversible reactions. Glycogen synthase is active when dephosphorylated; glycogen phosphorylase is active when phosphorylated by a cyclic AMPdependent protein kinase (incorrect answers a, c-e). These enzyme phosphorylations and dephosphorylations integrate glycogen synthesis/breakdown with food and glucose availability (refer to Fig. 8 in the High-Yield Facts). Glycogen storage diseases are a group of inherited enzyme deficiencies that cause accumulation of glycogen in liver, heart, or muscle. Glucose is the primary source of energy for most cells and excess glucose is stored as glycogen. Glycogen provides for short-term high-energy consumption in muscle and is an emergency energy supply for the brain. Glycogen stored in the liver can be converted back to glucose for release into the blood stream for use by other tissues. Deficiency of adenylyl kinase or cAMP-dependent protein kinase can alter glycogenesis/glycogenolysis regulation and produce glycogen storage

Question 10

An 11-month-old infant with hypoglycemia and a palpable liver is evaluated for possible glycogen storage disease. The parents have immigrated from Russia, and report that the child’s older brother was diagnosed with a “debrancher” enzyme deficiency with similar glycogen storage. This diagnosis would imply accumulation of glycogen with which type of glucose linkages?

A	Linear α1 → 4 linkages with branching α1 → 6 linkages
B	Linear α1 → 6 linkages with branching β1 → 4 linkages
C	Linear β1 → 4 linkages only
D	Linear β1 → 6 linkages only
E	Branching β1 → 6 linkages only

Question 10 Explanation:

Normal glycogen is composed of glucose residues joined in straight chains by β1 → 4 linkages. At 4- to 10-residue intervals, a branch of β1 → 4 linkages is initiated at a β1 → 6 linkage (incorrect answers b-e). Glycogen particles can contain up to 60,000 glucose residues. In the absence of the debrancher enzyme, glycogen can be degraded only to the branch points, inhibiting release of glucose into the serum and causing glycogen storage

Question 11

A 7-year-old obese Hispanic girl presents with dehydration after 3 days of vomiting and diarrhea. Her parents mention that a sibling was diagnosed with a type of diabetes that spilled sugar into the urine but did not need treatment. Urine reagent strip test for reducing sugars is strongly positive. The physician obtains a blood glucose level that is normal and a urine glucose oxidase test on the urine is also negative for glucose. Further analysis of the urine reveals a small amount of fructose and a large amount of an unidentified pentose that is most likely which of the following?

A	Galactose
B	Glucose
C	Lactose
D	Mannose
E	Xylulose

Question 11 Explanation:

It is important to differentiate glucosuria due to diabetes mellitus or renal tubular problems from other sugars in the urine, like galactose in galactosemia or fructose/xylulose in essential fructosuria (all these are reducing sugars that are positive with Clinitest but only glucose is positive with the glucose oxidase reagent strip Dextrostix—the test is nonspecific like dipstick tests for hemoglobin/myoglobin). The uronic acid pathway, like the pentose phosphate pathway, provides an alternate fate for glucose without generating ATP. Glucose 6-phosphate is converted to glucose 1-phosphate and reacted with UTP to form the higher energy compound UDP-glucose. UDP-glucose is converted to UDP-glucuronic acid that is a precursor for glucuronide units in proteoglycan polymers. Unused glucuronic acid is converted to xylulose (incorrect answers a-d) and then to xylitol by a xylulose reductase, the enzyme deficiency in essential pentosuria (MIM*260800). In this “disease,” which is better called a trait, excess xylulose is excreted into urine but causes no pathology. Pentoses (5-carbon sugars) are important in the pentose phosphate and uronic acid pathways, providing ribose for nucleic acid metabolism. The other sugars listed as options except for lactose are 6-carbon hexoses.

Question 12

A 2-week-old Caucasian boy returns to his pediatrician for evaluation of increased jaundice during his nursery stay and to follow up on a positive newborn screen for galactosemia (MIM*230400). Clinical assessment suggests the child’s jaundice has resolved and he has a good weight gain with no feeding concerns. The pediatrician obtains a repeat newborn screen, but recalls that certain carbohydrates can be recognized as reducing substances in urine by the Clinitest reaction that produces a green color. The urine reagent strip test is positive and physician suggests a switch to nonlactose formula until the nurse mentions that she spilled the urine and performed the test on a tabletop where sugar for coffee had been spilled. Among the following C6 isomers of glucose, which is a ketose and reducing substance?

A	Fructose
B	Galactose
C	Glucofuranose
D	Glucopyranose
E	Mannose

Question 12 Explanation:

6-Carbon hexoses with a C1 aldehyde group and four asymmetric carbons can generate 16 isomeric forms including ketoses with a C2 ketone group (eg, fructose—incorrect answers b-e) and aldoses with a C1 aldehyde group (glucose, galactose, and mannose). All are reducing sugars that will give a positive reducing substance reaction by urine dipstick, including fructose from the sucrose in table sugar. Glucofuranose and glucopyranose are ring structures of glucose, with the majority of glucose in solution in the glucopyranose form.

Question 13

Which of the following explains why individuals with hyperlipidemia and/or gout should minimize their intake of sucrose and high-fructose syrups?

A	Fructose can bypass phosphofructokinase using a fructokinase in liver.
B	Fructose can be phosphorylated by hexokinase in liver cells.
C	Fructose is converted to UDP-fructose in liver.
D	Fructose is ultimately converted to galactose in liver.
E	Fructose can be phosphorylated by hexokinase in adipose cells.

Question 13 Explanation:

Fructose is taken in by humans as sucrose, sucrose-containing syrups, and the free sugar. In liver, fructose is phosphorylated to fructose 1-phosphate by liver fructokinase, allowing it to bypass the ATP-regulated phosphofructokinase, yield glyceraldehyde and dihydroxyacetone phosphate by aldolase cleavage, and increase triglyceride/lipid biosynthesis (incorrect answers b-d). Fructose phosphorylation can also deplete liver cell ATP, lessening its inhibition of adenine nucleotide degradation and increasing uric acid that is the problem in gout. In adipocytes, fructose can be alternatively phosphorylated by hexokinase to fructose 6-phosphate. However, this reaction is competitively inhibited by appreciable amounts of glucose, as it is in other tissues.

Question 14

A 14-year-old Caucasian adolescent with past history of drug/alcohol abuse is brought to the emergency room after she cannot be aroused for school; her parents think that a bottle of acetaminophen (Tylenol) in their medicine cabinet has fewer tablets than they remember. The ER physician notes that cellulose is listed as the solid binder in the particular brand of acetaminophen. Since liver-protective N-acetylcysteamine therapy must be started within 16 hours of potential toxicity, the physician suggests that a rapid test for cellulose ingestion would be helpful in ascertaining if the child took the acetaminophen. Which of the following tests would be most informative regarding cellulose ingestion?

A	Serum glucose to reflect intestinal digestion and absorption of cellulose
B	Serum glucose after hydrolysis to break up cellulose β1 → 4 bonds
C	Stool glucose after hydrolysis to break up cellulose β1 → 4 bonds
D	Stool glucose to reflect intestinal digestion of ingested cellulose
E	Stool glucose to reflect intestinal digestion of β1 → 4 bonds distal to β1 → 6 branching points in cellulose

Question 14 Explanation:

Glucose (glucopyranose) residues in cellulose are linked by β1 → 4 bonds in straight chains that humans cannot hydrolyze because they do not possess an enzyme to carry out this function. Cellulose is a structural constituent of plants that is insoluble and provides a source of fiber in the diet. Humans do have intestinal lactase to cleave galactose β1 → 4 glucose (lactose) bonds, maltase to cleave glucose β1 → 4 glucose (maltose) bonds, and sucrose to cleave glucose β1 → 4 fructose (sucrose) bonds. N-acetylcysteamine therapy must be started within 8 hours of acetaminophen ingestion to prevent and within 16 hours to ameliorate liver toxicity, a severe potentially lethal result if over 10 times the therapeutic dose of 5 mg/kg acetaminophen is ingested.

Question 15

A frequent presentation in the newborn period is transient hypoglycemia as the child adapts to separation from maternal glucose controls. Blood glucose is generally maintained at concentrations of 4.5 to 5.5 mmol/L but may rise to 6.5 to 7.2 mmol/L after feeding or decrease to 3.3 to 3.9 mmol/L in the fasting state. Which of the following enzymes plays an important role in regulating blood glucose levels after feeding?

A	Glucokinase
B	Glucose-6-phosphatase
C	Phosphofructokinase
D	Pyruvate kinase
E	Glucose-6-phosphate dehydrogenase

Question 15 Explanation:

Glucokinase promotes uptake of large amounts of glucose by the liver while the other enzymes are present in pathways active after glucose uptake (incorrect answers b-e). At normal glucose levels, the liver produces glucose from glycogen, but as glucose levels rise after feeding, the liver stops converting glycogen and instead takes up glucose. Insulin also plays a role in regulating blood glucose levels. Pancreatic β-cells produce insulin in response to hyperglycemia. Glucose uptake by the β-cells and phosphorylation by glucokinase stimulate secretion of insulin, which enhances glucose transport into adipose tissues and muscle and thus lowers blood glucose levels.

Question 16

A 5-year-old African American girl is brought to the emergency room with tonic-clonic seizures. Laboratory studies are drawn as anticonvulsants given do not completely stop the A blood glucose of 30 mg/dL (normal 60-100) is found, prompting administration of 25% dextrose intravenously and 1 mg of glucagon intramuscularly. Which of the following events will occur in response to glucagon?

A	CO2 is consumed.
B	Inorganic phosphate is consumed.
C	Acetyl-CoA is utilized.
D	ATP is generated.
E	GTP is generated.

Question 17

A 6-month-old Caucasian boy has exhibited somewhat slow growth but becomes very ill after contracting influenza from an older sibling. A plasma lactate level of 55 mg/dL is found that suggests underlying mitochondrial disease, and mitochondrial DNA studies confirm a large deletion that affects several oxidative- phosphorylation complexes. Exacerbation of routine illness by the child’s reduced energy reserves would most affect which of the following reactions of the citric acid cycle?

A	Citrate → α-ketoglutarate
B	α-Ketoglutarate → succinate
C	Succinate → fumarate
D	Fumarate → malate
E	Malate → oxaloacetate

Question 17 Explanation:

During conversion of α-ketoglutarate to succinate, a molecule of GTP is synthesized from GDP and phosphate using energy from the hydrolysis of succinyl-CoA to produce succinate and CoA (incorrect answers a, c-e). This constitutes substrate-level phosphorylation, and, in contrast to oxidative phosphorylation, this is the only reaction in the citric acid cycle that directly yields a high-energy phosphate bond. The sequence of reactions from α-ketoglutarate to succinate is catalyzed by the α-ketoglutarate dehydrogenase complex and succinate thiokinase, respectively.

Question 18

A 45-year-old African American female is found unconscious at her desk and rushed to the emergency room via ambulance. She is a known diabetic, and an injectable insulin pen was found on her desk. Serum glucose was less than 20 g/dL as read by glucometer on the way to the hospital. Which of the following statements reflects the pathogenesis of her hypoglycemia?

A	Her liver was suddenly permeable to glucose after added insulin.
B	Her insulin increased glucose import into extrahepatic tissues using various glucose transporters.
C	Her insulin stimulated liver glucokinase, which phosphorylates glucose only at low glucose concentrations.
D	Her extrahepatic tissues became permeable to glucose through the opposing action of glucagon.
E	Insulin stimulated glucose uptake into her liver even when the serum glucose became low.

Question 18 Explanation:

Liver cells are permeable to glucose while extrahepatic tissues require insulin for glucose entry, reflecting different glucose transporters (GLUT) in different tissues (incorrect answers a, d, and e). Liver hexokinase has a low Km for glucose and acts at a constant rate, while glucokinase has a higher Km for glucose and promotes glucose uptake at high concentrations as found in the portal vein after meals (incorrect answer c). The liver releases glucose at normal serum glucose concentrations but takes up glucose at high serum glucose concentrations. Insulin and glucagon act in opposing fashion to regulate serum glucose concentration. Insulin, secreted by the pancreatic β-cell in response to internal increases in glucose, ATP, and calcium influx, increases glucose uptake by muscle and adipose cells by recruiting glucose transporters to their plasma membranes. Glucagon, secreted by the pancreatic α-cells, stimulates cyclic AMP synthesis with increased gluconeogenesis and glycogenolysis to increase serum glucose concentrations.

Question 19

Which of the following two compounds are the primary products of the pentose phosphate pathway?

A	NAD+ and ribose
B	NADH and ribose
C	NADP+ and ribose
D	NADPH and ribose
E	NAD+ and glucose
F	NADH and glucose
G	NADP+ and glucose
H	NADPH and glucose

Question 19 Explanation:

The pentose phosphate cycle does not produce ATP, but instead produces ribose and NADPH. NADP+ is the hydrogen acceptor instead of NAD+, as in glycolysis. In the oxidative phase of the pentose phosphate pathway, NADPH is generated by glucose-6- phosphate dehydrogenase. NADPH is also generated by 6-phosphogluconate dehydrogenase. Ribose is generated in the nonoxidative phase

Question 20

Which of the following is an energy-requiring step of glycolysis?

A	Glucokinase
B	Lactate dehydrogenase
C	Phosphoglycerate kinase
D	Pyruvate kinase
E	Phosphohexose isomerase

Question 20 Explanation:

Glucokinase catalyzes the conversion of glucose to glucose 6-phosphate in the energy, requiring first step of glycolysis. ATP is also required in the conversion of fructose 6-phosphate to fructose 1,6-bisphosphate by phosphofructokinase. ATP is generated in the conversion of 1,3-bisphos-phoglycerate to 3-phosphoglycerate by phosphoglycerate kinase and in the conversion of phosphoenolpyruvate to pyruvate by pyruvate kinase (incorrect answers c-d). No energy is required when lactate dehydrogenase converts pyruvate to lactate (a molecule of NADH is converted to NAD+-incorrect answer b) or when phosphohexose isomerase converts glucose 6-phosphate to fructose 6-phosphate (incorrect answer e).

Question 21

Which of the following are primary substrates for gluconeogenesis?

A	Galactose and fructose
B	Glycerol and alanine
C	Acetyl-CoA and succinyl-CoA
D	Sucrose and lactose
E	GTP and biotin

Question 21 Explanation:

Gluconeogenesis refers to the pathway for converting noncarbohydrate precursors to glucose (incorrect answers a, d). Glycerol, lactate, propionate, and certain amino acids such as alanine are all substrates for gluconeogenesis (substrates will undergo chemical conversion to form carbohydrates rather than serving as regulatory factors, incorrect answers a, d). Biotin is a cofactor for the first step (pyruvate carboxylase) of gluconeogenesis, while conversion of succinyl-CoA to acetyl-CoA provides GTP for the second step (phosphoenolpyruvate carboxykinase—incorrect answers c, e). Glycogen is a glucose storage molecule that can readily be converted in the liver back to glucose for maintenance of blood glucose levels between meals.

Question 22

A 3-month-old Caucasian girl presents with low blood glucose (hypoglycemia), enlarged liver (hepatomegaly), and excess fat deposition in the cheeks (cherubic facies). A liver biopsy reveals excess glycogen in hepatocytes. Deficiency of which of the following enzymes best explains this phenotype?

A	α-1,1-glucosidase
B	α-1,1-galactosidase
C	α-1,4-glucosidase
D	α-1,4-galactosidase
E	α-1,6-galactosidase

Question 22 Explanation:

The child has symptoms of glycogen storage disease. Glycogen is a glucose polymer with linear regions linked through the C1 aldehyde of one glucose to the C4 alcohol of the next (α-1,4-glucoside linkage). There are also branches from the linear glycogen polymer that have α-1,6-glucoside linkages. Glycogen is synthesized during times of carbohydrate and energy surplus, but must be degraded during fasting to provide energy. Separate enzymes for breakdown include phosphorylases (α-1,4-glucosidases) that cleave linear regions of glycogen and debranching enzymes (α-1,6-glucosidases) that cleave branch points. Glucose-6-phosphatase is needed in the liver to liberate free glucose from glucose 6-phosphate, providing fuel for other organs. There is no glucose-6-phosphatase in muscle, and muscle glycogenolysis provides energy just for muscle with production of lactate. Deficiencies of more than eight enzymes involved in glycogenolysis, including those mentioned, can produce glycogen storage disease.

Question 23

An 18-year-old Hispanic female is evaluated for fainting spells that tend to occur when she has not eaten due to illness or work. Her body mass index [weight in kg/(height in m)2] is over 29 (<26 desirable) and the initial history makes anxiety or blood pressure changes unlikely. Several random blood samples demonstrate lower glucose than normal and a glucose tolerance test is abnormal, showing a slow decrease in administered glucose that is suggestive of diabetes mellitus. Glucagon proves effective in helping her fainting spells, and a dietician places her on a moderate, low-carbohydrate diet, with monitoring of glucose by glucometer and use of glucagon when symptomatic. What is the role of glucagon in this situation?

A	To stimulate the citric acid cycle when tissue resistance leads to excess insulin
B	To stimulate gluconeogenesis when tissue resistance leads to excess insulin
C	To stimulate glycolysis in the presence of decreased insulin secretion
D	To stimulate the pentose phosphate pathway in the presence of decreased insulin secretion
E	To stimulate glycogenolysis in the presence of decreased insulin secretion

Question 23 Explanation:

Glucagon responds to decreases in blood glucose by increasing liver cAMP, gluconeogenesis, and glycogenolysis. High cAMP levels activate cAMP-dependent protein kinases that inactivate glycolytic enzymes pyruvate kinase and phosphofructokinase and activate glycogen phosphorylase by phosphorylation (incorrect answers a, c, and d). Obesity most commonly causes type II diabetes mellitus due to tissue resistance rather than decreased insulin secretion by the pancreas (incorrect answer e). Early stages of diabetes mellitus may be accompanied by periodic hypoglycemia due to high insulin levels trying to overcome tissue resistance or to periodic bursts of insulin secretion from failing pancreatic β-cells. Glucagon antagonizes insulin by increasing gluconeogenesis and glycogenolysis, both by increased action of cAMP kinases and through allosteric regulation of enzymes—for example, reciprocal activation of pyruvate carboxylase (gluconeogenesis) and inactivation of pyruvate dehydrogenase (glycolysis) by acetyl-CoA. Glucagon also inhibits lipolysis.

Question 24

A 2-year-old African American boy is evaluated for short stature (height below the 3rd percentile with weight and head circumference at the 20th percentile for age), irritability between meals accompanied by low glucose levels, enlarged liver, and easy bruisability. Liver biopsy reveals excess glycogen and deficiency of the α1 → 6 branching enzyme is documented for a diagnosis of type IV glycogen storage disease (MIM*232500). This disorder causes variable liver and neuromuscular symptoms but can be lethal due to progressive liver scarring (cirrhosis) because of the presence of abnormal glycogen. Conventional therapy with continuous glucose supply (eg, nocturnal cornstarch by tube) to minimize the need for glycogen breakdown is often not effective, and additional agents to minimize synthesis of abnormal glycogen have been considered. Inhibition of glycogen synthesis might target which of the following molecules?

A	UDP-glucose 1-phosphate
B	UDP-glucose
C	UDP-glucose 6-phosphate
D	Glucose 6-phosphate
E	Glucose 1-phosphate

Question 24 Explanation:

UDP-glucose is the high-energy compound from which glycogen is synthesized, made by condensing glucose-1-P and UTP using UDP-glucose pyrophosphorylase (incorrect answers a, c-e). Glucose is rapidly converted to glucose 6-phosphate by glucokinase in liver, its high Km favoring the high concentrations of glucose in the portal vein (route from glucose-absorbing intestinal veins to the liver). Glucose 6- phosphate is converted to glucose 1-phosphate and activated to form UDP-glucose, a molecule that can arise from UDP-galactose through lactose absorption or follow the glucuronic acid pathway to produce ascorbic acid (vitamin C) and building blocks for proteoglycans. UDP-glucose is added to terminal glucose units of glycogen byglycogen synthase in the form of α1 → 4 linkages. To increase the solubility of glycogen and to increase the number of terminal residues, glycogen-branching enzyme transfers blocks of ∼7 glucose residues to branch points via α1 → 6 linkages. Absence of the branching enzyme, deficient in type IV glycogen storage disease (MIM*232500), produces a less soluble glycogen with fewer termini, thus decreasing glucose release (short stature from less energy, large liver with stored glycogen) and provoking reaction to the abnormal glycogen to cause liver cell death (scarring and cirrhosis).

Question 25

Which of the following statements best describes the structure of glycogen?

A	Glycogen is a copolymer of glucose and galactose.
B	There are more branch residues than residues in straight chains.
C	Branch points contain α1-4 glycosidic linkages.
D	New glucose molecules are added to the C1 aldehyde group of chain termini, forming a hemiacetal.
E	The monosaccharide residues alternate between D- and L-glucose.

Question 25 Explanation:

Glycogen is a highly branched polymer of α-d-glucose residues joined by α1 → 4-glycosidic linkage. Under the influence of glycogen synthase, the C4 alcohol of a new glucose is added to the C1 aldehyde group of the chain terminus. The branched chains occur about every 10 residues and are joined in α1 → 6-glycosidic linkages. Large amounts of glycogen are stored as 100- to 400-Å granules in the cytoplasm of liver and muscle cells. The enzymes responsible for making or breaking the α1 → 4-glycosidic bonds are contained within the granules. Thus, glycogen is a readily mobilized form of glucose.

Question 26

A 25-year-old African American male notices increased fatigue and decreased performance when playing outfield for his office softball team. After a game in hot weather with many fielding chances, he has severe muscle pains and cramps that keep him awake and notices that his urine is dark that night. His physician suspects McArdle disease (type V glycogen storage disease—MIM*232600) and has him run on a treadmill breathing air with reduced oxygen. The man develops severe cramps after this ischemic exercise, further supporting the presumptive diagnosis. A diagnostic muscle biopsy is likely to show which of the following?

A	Increased normal glycogen with deficient hexokinase
B	Increased normal glycogen with deficient glycogen synthase
C	Increased normal glycogen with deficient phosphorylase
D	Decreased muscle glycogen with decreased glycogen synthase
E	Decreased muscle glycogen with decreased debranching enzyme

Question 26 Explanation:

Glycogen storage diseases are caused by abnormal glycogen breakdown with hepatic (hypoglycemia, hepatomegaly, and short stature) or muscle (exercise fatigue, cramping) symptoms. The excess glycogen (incorrect answers d, e) usually reflects defective glycogenolytic enzymes or their phosphorylating kinases but some defects of glycogen synthesis (like the type IV with deficient branching enzyme) or alterations of regulatory enzymes like phosphofructokinase can lead to decreased glycogen mobilization (answers d, e incorrect). Types V (phosphorylase deficiency) and VII (phosphofructokinase deficiency of Tarui disease) are due to enzymes with major effects on muscle while others (I, III-VIII) affect enzymes with major effects on liver (some like Type IV can affect both). The compromised phosphorylation of muscle glycogen characteristic of McArdle disease compels the muscles to rely on auxiliary energy sources such as free fatty acids and ambient glucose.

Question 27

The 10-year experience of a biochemical genetics laboratory includes five patients who had a positive urine reagent dipstick reaction for reducing sugars. Two of these were young females evaluated for vomiting and failure to thrive, while three were a boy and two girls, who were studied as part of a control population. Mass spectrometry profiles had shown a urine chemical with six carbons and a ketone group. Which of the following metabolites satisfy these chemical criteria and accumulate in specific inborn errors?

A	Fructose, fructose 1-phosphate
B	Galactose, galactose 1-phosphate
C	Glucose, glucose 1-phosphate
D	Ribulose, ribose 1-phosphate
E	Xylulose, xylose 1-phosphate

Question 27 Explanation:

Fructose is a 6-carbon hexose with a ketone group (ketose), while ribulose and xylulose have 5 carbons (incorrect answers d, e) and glucose and galactose have aldehyde rather than ketone groups (incorrect answers b, c). Fructose is the substrate for fructokinase that is deficient in asymptomatic fructosuria (MIM*229800), while fructose 1-phosphate is the substrate for aldolase B that is deficient in hereditary fructose intolerance (MIM*229600) with vomiting, lactic acidosis, and, with continued fructose ingestion, failure to thrive and hepatic disease. Glucose isomers can have different orientations of the H and OH groups on the carbon atom adjacent to the CH2OH group (D- vs L-isomers), and most of the monosaccharides in mammals are in the D form but differ in their configuration of the H and OH groups at the 2, 3, and 4 carbons (epimers). Mannose and galactose are the most biologically important epimers of glucose.

Question 28

A 17-year-old Caucasian adolescent is evaluated for fainting spells and found to have a glucose level of 54 mg/dL (normal 75-105) one hour after eating. Her body mass index (BMI) is 17 (average 23-25), and physical examination reveals pallor, halitosis, and discolored teeth with multiple caries. An eating disorder is suspected, and she enlists in a research study that includes behavioral and dietary therapy along with metabolic studies. Administration of deuterium-labeled glucose shows initial rapid entry into liver and peripheral muscle followed by continued muscle but minimal liver intake. Which of the following accurately interprets these results?

A	Abnormal liver uptake reflecting starvation
B	Abnormal liver uptake reflecting latent diabetes mellitus
C	Abnormal liver uptake reflecting prior glycogen depletion
D	Normal liver uptake reflecting a lower Km for liver hexokinase
E	Normal liver uptake reflecting a higher Km for liver glucokinase

Question 28 Explanation:

Less glucose uptake by liver after postprandial glucose levels fall is normal (incorrect answers ac) and does not reflect depleted glycogen or starvation from the patient’s likely bulimia (thin tooth enamel and decay from frequent induced vomiting). Most peripheral tissues such as muscle contain hexokinase with a low Km and high affinity for glucose that brings it rapidly into cells. After a meal, glucose absorbed from the intestine will produce high levels in serum that decreases rapidly as extrahepatic tissues import and convert glucose to glucose-6-phophate with hexokinase. In contrast, liver has a different hexokinase enzyme called glucokinase (incorrect answer d) that has a high Km and low affinity, removing glucose from the portal vein only when it is at high concentrations (shortly after a meal). This ensures that the liver will sequester glucose only when it is in excess and not needed by tissues such as brain or heart, storing the excess glucose as glycogen for later use.

Question 29

The figure below depicts the initial steps of glycolysis with enzyme reactions A and E, and substrates B, C, and D. Which of the following options depicts the correct reaction sequence: 1—a reaction regulating glucose levels, 2—endergonic reaction(s), requiring ATP to ADP coupling, and 3—reaction(s), producing a substrate that will be reduced to produce NADH?

A	1-reaction A, 2-reactions A and B to C, 3-reactions C to D and E
B	1-reaction A, 2-reactions A and C to D, 3-reactions C to D and E
C	1-reaction A, 2-reaction B to C, 3-reactions B to C and E
D	1-reaction E, 2-reaction A, 3-reactions C to D and E
E	1-reaction E, 2-reaction E, 3-reactions C to D and E

Question 29 Explanation:

Glucose is converted to glucose 6-phosphate (G6P) by hexokinase (reaction A in figure below Question 94) in most tissues. However, liver that has a glucokinase with higher Km that ensures glucose import and conversion to glycogen occurs only during periods of glucose excess, that is after meals (incorrect answers d, e—see prior Answer 93 for more detail). Hexo- or glucokinase (reaction A in figure) and phosphofructokinase converting fructose 6-phosphate to fructose 1,6-bisphosphate (reaction B-C in figure) both are endergonic reactions that require energy and are coupled with exergonic ATP to ADP hydrolysis to allow conversion (incorrect answers b-e). This energy deficit is made up later in glycolysis by the exergonic phosphoglycerate kinase and pyruvate kinase reactions that generate ATP (four per molecule of glucose), entering glycolysis and giving a net yield of 2 ATP for each conversion of glucose to pyruvate. Glyceraldehyde 3-phosphate (substrate D in figure) is produced from fructose 1,6-diphos-phate (substrate C) or from dihydroxyacetone phosphate by phosphotriose isomerase (reaction E) and reduced to yield NADH (incorrect answer c).

Question 30

A 14-year-old African American adolescent competes well in short distance events as she tries out for the track team but does poorly at longer distances. Evaluation at a sports institute shows she has elevation of blood lactate after longer periods on a treadmill and suggests a defect in muscle energy metabolism. Although the girl’s parents decline further expensive evaluation, which of the following reactions of muscle metabolism would be least affected after she has run a long distance?

A	Glucose 6-phosphate to fructose 6-phosphate
B	Glucose to glucose 6-phosphate
C	Fructose 6-phosphate to fructose 1,6-diphosphate
D	Phosphoenolpyruvate to pyruvate
E	Pyruvate to lactate

Question 30 Explanation:

The indicated reactions are part of the glycolytic pathway and only the pyruvate kinase reaction (option d) is exergonic—produces energy that is coupled to produce ATP from ADP and Pi. Less effective energy production in muscle implied by the girl’s fatigue would affect this reaction less than others that are endergonic or energy neutral (incorrect answers a-c, e). Major defects affecting muscle glycogen degradation or mitochondrial electron transport will produce weakness/developmental delay at young ages or exercise intolerance (muscle aches and spasms) at later ages. Milder defects produce more subtle symptoms that can manifest as early fatigue or poor performance in high intensity or endurance sports. It is interesting to speculate if mild mutations enhancing muscle energy pathways will be found in exceptional athletes, but the only supplements that are proven to work are those that increase muscle mass (steroids, growth hormone) rather than “energy drinks” or supplements such as creatine.

Question 31

Which of the following enzymes catalyzes high-energy phosphorylation of substrates during glycolysis?

A	Pyruvate kinase
B	Phosphoglycerate kinase
C	Triose phosphate isomerase
D	Aldolase
E	Glyceraldehyde-3-phosphate dehydrogenase

Question 31 Explanation:

Using NAD+ in an oxidation-reduction reaction, inorganic phosphate is added to glyceraldehyde 3-phosphate by the enzyme glyceraldehyde-3-phosphate dehydrogenase to form 1,3-diphosphoglycerate. Other enzymes (incorrect answers a, b, and d) add highenergy phosphate bonds to substrates of glycolysis. Hexokinase—or, in the case of the liver, glucokinase—adds phosphate from ATP to glucose to form glucose 6- phosphate. Strictly speaking, this is not always considered a step of the glycolytic pathway. Phosphofructokinase uses ATP to convert fructose 6-phosphate to fructose 1,6-phosphate, while phosphoglycerate kinase and pyruvate kinase transfer substrate high-energy phosphate groups to ADP to form ATP. Triose phosphate isomerase interconverts dihydroxyacetone phosphate and glyceraldehyde 3-phosphate without phosphorylation (incorrect answer c).

Question 32

Which reaction in the figure below occurs in both muscle and liver but has substantially different qualities in the two?

A	Reaction A
B	Reaction B
C	Reaction C
D	Reaction D
E	Reaction E

Question 32 Explanation:

The conversion of glucose to glucose 6-phosphate (reaction B in figure) is different in liver and muscle, while others are similar (incorrect answers a, c-e). In muscle and most other tissues, hexokinase regulates the conversion of glucose to glucose 6-phosphate. When the major regulatory enzyme of glycolysis, phosphofructose kinase, is turned off, the level of fructose 6-phosphate increases and, in turn, the level of glucose 6- phosphate rises because it is in equilibrium with fructose 6-phosphate. Hexokinase is inhibited by glucose 6-phosphate. However, in the liver, glucose is phosphorylated even when glucose 6-phosphate levels are high because the enzyme regulating transformation of glucose into glucose 6-phosphate is glucokinase. Glucokinase is not inhibited by glucose 6-phosphate in the liver. Although hexokinase has a low Km for glucose and is capable of acting on low levels of blood glucose,glucokinase has a high Km for glucose and is effective only when glucose is abundant. Therefore, when blood glucose levels are low, muscle, brain, and other tissues are capable of taking up and phosphorylating glucose, whereas the liver is not. When blood glucose is abundant, glucokinase in the liver phosphorylates glucose and provides glucose 6-phosphate for the synthesis and storage of glucose as glycogen.

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1. Diarrhea from infection or malnutrition is the world’s most prevalent killer of A 2-month-old Caucasian girl develops chronic diarrhea and liver inflammation in early infancy when the mother begins using formula that includes corn syrup. Evaluation of the child demonstrates sensitivity to fructose in the diet. Which of the following glycosides contains fructose and therefore should be avoided when feeding or treating this infant?

Sucrose
Ouabain
Lactose
Maltose
Streptomycin

2. Which of the following carbohydrates would be most abundant in the diet of strict vegetarians?

Amylose
Lactose
Cellulose
Maltose
Glycogen

3. Alcohol abuse affects over 13% of adults in the United States, costing more health care dollars ($184 billion annually) than cancer ($107 billion) or obesity ($100 billion) in some studies. In 2001, 47% of those between ages 12 and 20 years reported drinking, 30% of these to binge drinking in the last month. Chronic alcoholics require more ethanol than do nondrinkers to become intoxicated because of a higher level of a specific enzyme. However, independent of specific enzyme levels, the availability of what other substance is rate-limiting in the clearance of ethanol?

NADH
NAD+
FADH
FAD+
NADPH

4. In lung diseases such as emphysema or chronic bronchitis, there is chronic hypoxia that is particularly obvious in vascular tissues such as the lips or nail beds (cyanosis). Certain genetic diseases such as α1-antitrypsin deficiency (MIM*107400) predispose to emphysema, as do environmental exposures such as cigarette smoking or asbestos. Poorly perfused areas exposed to chronic hypoxia have decreased metabolic energy for tissue maintenance and repair. Which of the following is an important reason for this?

Increased hexokinase activity owing to increased oxidative phosphorylation
Increased ethanol formation from pyruvate on changing from anaerobic to aerobic metabolism
Increased glucose utilization via the pentose phosphate pathway on changing from anaerobic to aerobic metabolism
Decreased ATP generation and increased glucose utilization on changing from aerobic to anaerobic metabolism
Decreased respiratory quotient on changing from carbohydrate to fat as the major metabolic fuel

5. An 18-year-old Korean college student attends a fraternity party and is embarrassed because he becomes flushed and sick to his stomach after his first drink of He learns that this reaction is due to genetic variation in some Asians and Native Americans that affects metabolism of a metabolite of alcohol. Which of the following is the variably degraded metabolite?

Methanol
Acetone
Acetaldehyde
Hydrogen peroxide
Glycerol

6. A 6-month-old Caucasian girl is hospitalized for evaluation of short stature, enlarged liver, and intermittent lethargy/irritability that, on a recent emergency room visit, was accompanied by low blood glucose (hypoglycemia). In the same ward is a 6-year-old African American boy who is being evaluated for severe muscle cramping that raised suspicions for sickle cell The family history is unremarkable for these children; each has a normal sibling and normal parents. Both children are given diagnoses of glycogen storage disease with the infant girl having type I (MIM*232200) affecting liver and the older boy type V (McArdle disease, MIM*232600) affecting mainly muscle. Which of the following conversions explains the difference in these presentations?

Glycogen to lactate in liver
Glycogen and lactate to glucose in liver
Glycogen to glucose in muscle
Glycogen to alanine in muscle
Glycogen to glucose 6-phosphate in liver

7. A 3-day-old boy from an Iraqi refugee family exhibits severe tissue swelling (edema), rapid heart rate (tachycardia), enlargement of liver and spleen (hepatosplenomegaly), and jaundice of the eyes and skin. The couple had two prior female infants who are alive and well, and the wife relates that she lost a brother in infancy with severe hemolysis induced after a viral Blood tests show hemoglobin of 5 g/dL (normal neonate mean 18.5) with reticulocyte count of 15% (normal 1.8-4.6). Exchange transfusion is attempted but the child suffers cardiac arrest. Postmortem assay of red cell enzymes confirms the suspected diagnosis of glucose-6-phosphate dehydrogenase deficiency (MIM*305900), implying defective synthesis of which of the following compounds?

Deoxyribose and NADP
Glucose and lactate
Lactose and NADPH
Ribose and NADPH
Sucrose and NAD

8. Which of the following best explains why fructose was formerly recommended for patients with diabetes mellitus?

Fructose is a better substrate for hexokinase.
Fructose stimulates residual insulin release.
Fructose has a specific kinase in liver that allows bypass of phosphofructokinase.
Fructose is phosphorylated and cleaved to triose phosphates, which cannot be used for gluconeogenesis.
Hexokinase phosphorylates fructose in extrahepatic tissues, and its activity will not be affected by high glucose concentrations in diabetes.

9. A 2-month-old Caucasian girl is evaluated for hypoglycemia and lactic acidosis and noted to have an enlarged Biopsy reveals stored glycogen, and a glycogen storage disease is suspected. Assay of usual glycogen enzyme deficiencies in the liver specimen is normal, so the metabolic consultant recommends assay of rarer enzyme deficiencies that influence glycogen metabolism. These enzymes would most likely include which of the following?

Hexokinase
cAMP-dependent protein kinase
Glucose-6-phosphate dehydrogenase
Phosphofructokinase
Fructose-1,6-diphosphatase

10. An 11-month-old infant with hypoglycemia and a palpable liver is evaluated for possible glycogen storage disease. The parents have immigrated from Russia, and report that the child’s older brother was diagnosed with a “debrancher” enzyme deficiency with similar glycogen This diagnosis would imply accumulation of glycogen with which type of glucose linkages?

Linear α1 → 4 linkages with branching α1 → 6 linkages
Linear α1 → 6 linkages with branching β1 → 4 linkages
Linear β1 → 4 linkages only
Linear β1 → 6 linkages only
Branching β1 → 6 linkages only

11. A 7-year-old obese Hispanic girl presents with dehydration after 3 days of vomiting and Her parents mention that a sibling was diagnosed with a type of diabetes that spilled sugar into the urine but did not need treatment. Urine reagent strip test for reducing sugars is strongly positive. The physician obtains a blood glucose level that is normal and a urine glucose oxidase test on the urine is also negative for glucose. Further analysis of the urine reveals a small amount of fructose and a large amount of an unidentified pentose that is most likely which of the following?

Galactose
Glucose
Lactose
Mannose
Xylulose

12. A 2-week-old Caucasian boy returns to his pediatrician for evaluation of increased jaundice during his nursery stay and to follow up on a positive newborn screen for galactosemia (MIM*230400). Clinical assessment suggests the child’s jaundice has resolved and he has a good weight gain with no feeding The pediatrician obtains a repeat newborn screen, but recalls that certain carbohydrates can be recognized as reducing substances in urine by the Clinitest reaction that produces a green color. The urine reagent strip test is positive and physician suggests a switch to nonlactose formula until the nurse mentions that she spilled the urine and performed the test on a tabletop where sugar for coffee had been spilled. Among the following C6 isomers of glucose, which is a ketose and reducing substance?

Fructose
Galactose
Glucofuranose
Glucopyranose
Mannose

13. Which of the following explains why individuals with hyperlipidemia and/or gout should minimize their intake of sucrose and high-fructose syrups?

Fructose can bypass phosphofructokinase using a fructokinase in liver.
Fructose can be phosphorylated by hexokinase in liver cells.
Fructose is converted to UDP-fructose in liver.
Fructose is ultimately converted to galactose in liver.
Fructose can be phosphorylated by hexokinase in adipose cells.

14. A 14-year-old Caucasian adolescent with past history of drug/alcohol abuse is brought to the emergency room after she cannot be aroused for school; her parents think that a bottle of acetaminophen (Tylenol) in their medicine cabinet has fewer tablets than they The ER physician notes that cellulose is listed as the solid binder in the particular brand of acetaminophen. Since liver-protective N-acetylcysteamine therapy must be started within 16 hours of potential toxicity, the physician suggests that a rapid test for cellulose ingestion would be helpful in ascertaining if the child took the acetaminophen. Which of the following tests would be most informative regarding cellulose ingestion?

Serum glucose to reflect intestinal digestion and absorption of cellulose
Serum glucose after hydrolysis to break up cellulose β1 → 4 bonds
Stool glucose after hydrolysis to break up cellulose β1 → 4 bonds
Stool glucose to reflect intestinal digestion of ingested cellulose
Stool glucose to reflect intestinal digestion of β1 → 4 bonds distal to β1 → 6 branching points in cellulose

15. A frequent presentation in the newborn period is transient hypoglycemia as the child adapts to separation from maternal glucose Blood glucose is generally maintained at concentrations of 4.5 to 5.5 mmol/L but may rise to 6.5 to 7.2 mmol/L after feeding or decrease to 3.3 to 3.9 mmol/L in the fasting state. Which of the following enzymes plays an important role in regulating blood glucose levels after feeding?

Glucokinase
Glucose-6-phosphatase
Phosphofructokinase
Pyruvate kinase
Glucose-6-phosphate dehydrogenase

16. A 5-year-old African American girl is brought to the emergency room with tonic-clonic seizures. Laboratory studies are drawn as anticonvulsants given do not completely stop the A blood glucose of 30 mg/dL (normal 60-100) is found, prompting administration of 25% dextrose intravenously and 1 mg of glucagon intramuscularly. Which of the following events will occur in response to glucagon?

CO2 is consumed.
Inorganic phosphate is consumed.
Acetyl-CoA is utilized.
ATP is generated.
GTP is generated.

17. A 6-month-old Caucasian boy has exhibited somewhat slow growth but becomes very ill after contracting influenza from an older A plasma lactate level of 55 mg/dL is found that suggests underlying mitochondrial disease, and mitochondrial DNA studies confirm a large deletion that affects several oxidative- phosphorylation complexes. Exacerbation of routine illness by the child’s reduced energy reserves would most affect which of the following reactions of the citric acid cycle?

Citrate → α-ketoglutarate
α-Ketoglutarate → succinate
Succinate → fumarate
Fumarate → malate
Malate → oxaloacetate

18. A 45-year-old African American female is found unconscious at her desk and rushed to the emergency room via She is a known diabetic, and an injectable insulin pen was found on her desk. Serum glucose was less than 20 g/dL as read by glucometer on the way to the hospital. Which of the following statements reflects the pathogenesis of her hypoglycemia?

Her liver was suddenly permeable to glucose after added insulin.
Her insulin increased glucose import into extrahepatic tissues using various glucose transporters.
Her insulin stimulated liver glucokinase, which phosphorylates glucose only at low glucose concentrations.
Her extrahepatic tissues became permeable to glucose through the opposing action of glucagon.
Insulin stimulated glucose uptake into her liver even when the serum glucose became low.

19. Which of the following two compounds are the primary products of the pentose phosphate pathway?

NAD+ and ribose
NADH and ribose
NADP+ and ribose
NADPH and ribose
NAD+ and glucose
NADH and glucose
NADP+ and glucose
NADPH and glucose

20. Which of the following is an energy-requiring step of glycolysis?

Glucokinase
Lactate dehydrogenase
Phosphoglycerate kinase
Pyruvate kinase
Phosphohexose isomerase

21. Which of the following are primary substrates for gluconeogenesis?

Galactose and fructose
Glycerol and alanine
Acetyl-CoA and succinyl-CoA
Sucrose and lactose
GTP and biotin

22. A 3-month-old Caucasian girl presents with low blood glucose (hypoglycemia), enlarged liver (hepatomegaly), and excess fat deposition in the cheeks (cherubic facies). A liver biopsy reveals excess glycogen in Deficiency of which of the following enzymes best explains this phenotype?

α-1,1-glucosidase
α-1,1-galactosidase
α-1,4-glucosidase
α-1,4-galactosidase
α-1,6-galactosidase

23. An 18-year-old Hispanic female is evaluated for fainting spells that tend to occur when she has not eaten due to illness or work. Her body mass index [weight in kg/(height in m)2] is over 29 (<26 desirable) and the initial history makes anxiety or blood pressure changes Several random blood samples demonstrate lower glucose than normal and a glucose tolerance test is abnormal, showing a slow decrease in administered glucose that is suggestive of diabetes mellitus. Glucagon proves effective in helping her fainting spells, and a dietician places her on a moderate, low-carbohydrate diet, with monitoring of glucose by glucometer and use of glucagon when symptomatic. What is the role of glucagon in this situation?

To stimulate the citric acid cycle when tissue resistance leads to excess insulin
To stimulate gluconeogenesis when tissue resistance leads to excess insulin
To stimulate glycolysis in the presence of decreased insulin secretion
To stimulate the pentose phosphate pathway in the presence of decreased insulin secretion
To stimulate glycogenolysis in the presence of decreased insulin secretion

24. A 2-year-old African American boy is evaluated for short stature (height below the 3rd percentile with weight and head circumference at the 20th percentile for age), irritability between meals accompanied by low glucose levels, enlarged liver, and easy bruisability. Liver biopsy reveals excess glycogen and deficiency of the α1→ 6 branching enzyme is documented for a diagnosis of type IV glycogen storage disease (MIM*232500). This disorder causes variable liver and neuromuscular symptoms but can be lethal due to progressive liver scarring (cirrhosis) because of the presence of abnormal glycogen. Conventional therapy with continuous glucose supply (eg, nocturnal cornstarch by tube) to minimize the need for glycogen breakdown is often not effective, and additional agents to minimize synthesis of abnormal glycogen have been considered. Inhibition of glycogen synthesis might target which of the following molecules?

UDP-glucose 1-phosphate
UDP-glucose
UDP-glucose 6-phosphate
Glucose 6-phosphate
Glucose 1-phosphate

25. Which of the following statements best describes the structure of glycogen?

Glycogen is a copolymer of glucose and galactose.
There are more branch residues than residues in straight chains.
Branch points contain α1-4 glycosidic linkages.
New glucose molecules are added to the C1 aldehyde group of chain termini, forming a hemiacetal.
The monosaccharide residues alternate between D- and L-glucose.

26. A 25-year-old African American male notices increased fatigue and decreased performance when playing outfield for his office softball team. After a game in hot weather with many fielding chances, he has severe muscle pains and cramps that keep him awake and notices that his urine is dark that night. His physician suspects McArdle disease (type V glycogen storage disease—MIM*232600) and has him run on a treadmill breathing air with reduced oxygen. The man develops severe cramps after this ischemic exercise, further supporting the presumptive diagnosis. A diagnostic muscle biopsy is likely to show which of the following?

Increased normal glycogen with deficient hexokinase
Increased normal glycogen with deficient glycogen synthase
Increased normal glycogen with deficient phosphorylase
Decreased muscle glycogen with decreased glycogen synthase
Decreased muscle glycogen with decreased debranching enzyme

27. The 10-year experience of a biochemical genetics laboratory includes five patients who had a positive urine reagent dipstick reaction for reducing sugars. Two of these were young females evaluated for vomiting and failure to thrive, while three were a boy and two girls, who were studied as part of a control population. Mass spectrometry profiles had shown a urine chemical with six carbons and a ketone group. Which of the following metabolites satisfy these chemical criteria and accumulate in specific inborn errors?

Fructose, fructose 1-phosphate
Galactose, galactose 1-phosphate
Glucose, glucose 1-phosphate
Ribulose, ribose 1-phosphate
Xylulose, xylose 1-phosphate

28. A 17-year-old Caucasian adolescent is evaluated for fainting spells and found to have a glucose level of 54 mg/dL (normal 75-105) one hour after Her body mass index (BMI) is 17 (average 23-25), and physical examination reveals pallor, halitosis, and discolored teeth with multiple caries. An eating disorder is suspected, and she enlists in a research study that includes behavioral and dietary therapy along with metabolic studies. Administration of deuterium-labeled glucose shows initial rapid entry into liver and peripheral muscle followed by continued muscle but minimal liver intake. Which of the following accurately interprets these results?

Abnormal liver uptake reflecting starvation
Abnormal liver uptake reflecting latent diabetes mellitus
Abnormal liver uptake reflecting prior glycogen depletion
Normal liver uptake reflecting a lower Km for liver hexokinase
Normal liver uptake reflecting a higher Km for liver glucokinase

29. The figure below depicts the initial steps of glycolysis with enzyme reactions A and E, and substrates B, C, and Which of the following options depicts the correct reaction sequence: 1—a reaction regulating glucose levels, 2—endergonic reaction(s), requiring ATP to ADP coupling, and 3—reaction(s), producing a substrate that will be reduced to produce NADH?

1-reaction A, 2-reactions A and B to C, 3-reactions C to D and E
1-reaction A, 2-reactions A and C to D, 3-reactions C to D and E
1-reaction A, 2-reaction B to C, 3-reactions B to C and E
1-reaction E, 2-reaction A, 3-reactions C to D and E
1-reaction E, 2-reaction E, 3-reactions C to D and E

30. A 14-year-old African American adolescent competes well in short distance events as she tries out for the track team but does poorly at longer Evaluation at a sports institute shows she has elevation of blood lactate after longer periods on a treadmill and suggests a defect in muscle energy metabolism. Although the girl’s parents decline further expensive evaluation, which of the following reactions of muscle metabolism would be least affected after she has run a long distance?

Glucose 6-phosphate to fructose 6-phosphate
Glucose to glucose 6-phosphate
Fructose 6-phosphate to fructose 1,6-diphosphate
Phosphoenolpyruvate to pyruvate
Pyruvate to lactate

31. Which of the following enzymes catalyzes high-energy phosphorylation of substrates during glycolysis?

Pyruvate kinase
Phosphoglycerate kinase
Triose phosphate isomerase
Aldolase
Glyceraldehyde-3-phosphate dehydrogenase

32. Which reaction in the figure below occurs in both muscle and liver but has substantially different qualities in the two?

Reaction A
Reaction B
Reaction C
Reaction D
Reaction E

[MCQs] Carbohydrate Metabolism Quiz (32 test)

Carbohydrate Metabolism Quiz (32 test)

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