COMSAE PHASE 1 METABOLISM PRACTICE
EXAM WITH ACTUAL QUESTIONS AND
VERIFIED ANSWERS, PLUS EXPLAINED
RATIONALES/EXPERT VERIFIED FOR
GUARANTEED 100% PASS 2026/LATEST
UPDATE/INSTANT DOWNLOAD PDF
1. A 6-month-old infant presents with hepatomegaly, fasting
hypoglycemia, hyperuricemia, and lactic acidosis. Physical
examination reveals a protuberant abdomen and poor growth.
Which enzyme deficiency best explains these findings?
A. Acid alpha-glucosidase
B. Debranching enzyme
C. Branching enzyme
D. Glucose-6-phosphatase
Answer: D. Glucose-6-phosphatase
Rationale: This presentation describes Von Gierke disease (Glycogen
Storage Disease Type I). Deficiency of glucose-6-phosphatase prevents
conversion of glucose-6-phosphate into free glucose, causing severe
fasting hypoglycemia, glycogen accumulation in the liver, lactic
acidosis, hyperuricemia from competition with lactate for renal
excretion, and hyperlipidemia.
2. During prolonged fasting, which tissue contributes the greatest
amount of newly synthesized glucose through gluconeogenesis?
A. Skeletal muscle
B. Adipose tissue
1
,C. Liver
D. Brain
Answer: C. Liver
Rationale: The liver is the principal organ responsible for
gluconeogenesis during fasting. The kidney contributes significantly
during prolonged starvation, but hepatic gluconeogenesis predominates
initially.
3. A patient with pyruvate dehydrogenase deficiency is placed on a
ketogenic diet. Why is this dietary modification beneficial?
A. It increases glycolysis.
B. It stimulates glycogen synthesis.
C. It enhances pyruvate production.
D. It bypasses impaired carbohydrate oxidation by providing ketone
bodies.
Answer: D. It bypasses impaired carbohydrate oxidation by providing
ketone bodies.
Rationale: Pyruvate dehydrogenase deficiency prevents pyruvate from
entering the TCA cycle as acetyl-CoA. Ketogenic diets provide fat-
derived ketone bodies, supplying energy directly to the brain while
reducing pyruvate accumulation and lactate production.
4. Which enzyme is the primary rate-limiting enzyme of glycolysis?
A. Hexokinase
B. Pyruvate kinase
C. Glucokinase
D. Phosphofructokinase-1
Answer: D. Phosphofructokinase-1
2
,Rationale: PFK-1 is the committed and rate-limiting step of glycolysis. It
is activated by AMP and fructose-2,6-bisphosphate and inhibited by
ATP and citrate.
5. A marathon runner begins utilizing fatty acids as the primary
energy source after glycogen stores become depleted. Which
enzyme transports long-chain fatty acids into mitochondria?
A. Hormone-sensitive lipase
B. Acyl-CoA synthetase
C. Acetyl-CoA carboxylase
D. Carnitine palmitoyltransferase I
Answer: D. Carnitine palmitoyltransferase I
Rationale: CPT-I facilitates transport of long-chain fatty acids into
mitochondria for beta-oxidation. Malonyl-CoA inhibits CPT-I to prevent
simultaneous fatty acid synthesis and oxidation.
6. Which vitamin serves as the precursor of thiamine pyrophosphate?
A. Riboflavin
B. Niacin
C. Pyridoxine
D. Vitamin B1
Answer: D. Vitamin B1
Rationale: Thiamine (Vitamin B1) forms thiamine pyrophosphate, a
cofactor for pyruvate dehydrogenase, alpha-ketoglutarate
dehydrogenase, branched-chain alpha-ketoacid dehydrogenase, and
transketolase.
3
, 7. A child develops severe hypoglycemia after an overnight fast.
Laboratory studies reveal elevated medium-chain fatty acids and
absent ketones. Which enzyme is deficient?
A. CPT-I
B. Carnitine transporter
C. Very-long-chain acyl-CoA dehydrogenase
D. Medium-chain acyl-CoA dehydrogenase
Answer: D. Medium-chain acyl-CoA dehydrogenase
Rationale: MCAD deficiency causes impaired beta-oxidation of
medium-chain fatty acids, resulting in hypoketotic hypoglycemia during
fasting, elevated dicarboxylic acids, and sudden infant death risk.
8. Fructose-2,6-bisphosphate primarily functions by:
A. Activating gluconeogenesis
B. Inhibiting glycolysis
C. Stimulating glycogen breakdown
D. Activating phosphofructokinase-1
Answer: D. Activating phosphofructokinase-1
Rationale: Fructose-2,6-bisphosphate is a powerful activator of PFK-1
and inhibits fructose-1,6-bisphosphatase, promoting glycolysis while
suppressing gluconeogenesis.
9. Which amino acid cannot contribute to gluconeogenesis?
A. Alanine
B. Glutamine
C. Valine
D. Leucine
4
EXAM WITH ACTUAL QUESTIONS AND
VERIFIED ANSWERS, PLUS EXPLAINED
RATIONALES/EXPERT VERIFIED FOR
GUARANTEED 100% PASS 2026/LATEST
UPDATE/INSTANT DOWNLOAD PDF
1. A 6-month-old infant presents with hepatomegaly, fasting
hypoglycemia, hyperuricemia, and lactic acidosis. Physical
examination reveals a protuberant abdomen and poor growth.
Which enzyme deficiency best explains these findings?
A. Acid alpha-glucosidase
B. Debranching enzyme
C. Branching enzyme
D. Glucose-6-phosphatase
Answer: D. Glucose-6-phosphatase
Rationale: This presentation describes Von Gierke disease (Glycogen
Storage Disease Type I). Deficiency of glucose-6-phosphatase prevents
conversion of glucose-6-phosphate into free glucose, causing severe
fasting hypoglycemia, glycogen accumulation in the liver, lactic
acidosis, hyperuricemia from competition with lactate for renal
excretion, and hyperlipidemia.
2. During prolonged fasting, which tissue contributes the greatest
amount of newly synthesized glucose through gluconeogenesis?
A. Skeletal muscle
B. Adipose tissue
1
,C. Liver
D. Brain
Answer: C. Liver
Rationale: The liver is the principal organ responsible for
gluconeogenesis during fasting. The kidney contributes significantly
during prolonged starvation, but hepatic gluconeogenesis predominates
initially.
3. A patient with pyruvate dehydrogenase deficiency is placed on a
ketogenic diet. Why is this dietary modification beneficial?
A. It increases glycolysis.
B. It stimulates glycogen synthesis.
C. It enhances pyruvate production.
D. It bypasses impaired carbohydrate oxidation by providing ketone
bodies.
Answer: D. It bypasses impaired carbohydrate oxidation by providing
ketone bodies.
Rationale: Pyruvate dehydrogenase deficiency prevents pyruvate from
entering the TCA cycle as acetyl-CoA. Ketogenic diets provide fat-
derived ketone bodies, supplying energy directly to the brain while
reducing pyruvate accumulation and lactate production.
4. Which enzyme is the primary rate-limiting enzyme of glycolysis?
A. Hexokinase
B. Pyruvate kinase
C. Glucokinase
D. Phosphofructokinase-1
Answer: D. Phosphofructokinase-1
2
,Rationale: PFK-1 is the committed and rate-limiting step of glycolysis. It
is activated by AMP and fructose-2,6-bisphosphate and inhibited by
ATP and citrate.
5. A marathon runner begins utilizing fatty acids as the primary
energy source after glycogen stores become depleted. Which
enzyme transports long-chain fatty acids into mitochondria?
A. Hormone-sensitive lipase
B. Acyl-CoA synthetase
C. Acetyl-CoA carboxylase
D. Carnitine palmitoyltransferase I
Answer: D. Carnitine palmitoyltransferase I
Rationale: CPT-I facilitates transport of long-chain fatty acids into
mitochondria for beta-oxidation. Malonyl-CoA inhibits CPT-I to prevent
simultaneous fatty acid synthesis and oxidation.
6. Which vitamin serves as the precursor of thiamine pyrophosphate?
A. Riboflavin
B. Niacin
C. Pyridoxine
D. Vitamin B1
Answer: D. Vitamin B1
Rationale: Thiamine (Vitamin B1) forms thiamine pyrophosphate, a
cofactor for pyruvate dehydrogenase, alpha-ketoglutarate
dehydrogenase, branched-chain alpha-ketoacid dehydrogenase, and
transketolase.
3
, 7. A child develops severe hypoglycemia after an overnight fast.
Laboratory studies reveal elevated medium-chain fatty acids and
absent ketones. Which enzyme is deficient?
A. CPT-I
B. Carnitine transporter
C. Very-long-chain acyl-CoA dehydrogenase
D. Medium-chain acyl-CoA dehydrogenase
Answer: D. Medium-chain acyl-CoA dehydrogenase
Rationale: MCAD deficiency causes impaired beta-oxidation of
medium-chain fatty acids, resulting in hypoketotic hypoglycemia during
fasting, elevated dicarboxylic acids, and sudden infant death risk.
8. Fructose-2,6-bisphosphate primarily functions by:
A. Activating gluconeogenesis
B. Inhibiting glycolysis
C. Stimulating glycogen breakdown
D. Activating phosphofructokinase-1
Answer: D. Activating phosphofructokinase-1
Rationale: Fructose-2,6-bisphosphate is a powerful activator of PFK-1
and inhibits fructose-1,6-bisphosphatase, promoting glycolysis while
suppressing gluconeogenesis.
9. Which amino acid cannot contribute to gluconeogenesis?
A. Alanine
B. Glutamine
C. Valine
D. Leucine
4
