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NBME CBSE – 100 Practice Questions with Answers
Section 1: Biochemistry & Molecular Biology (Questions 1–15)
1. After 12 hours of fasting, which process is most increased in the liver to maintain blood
glucose?
A. Glycolysis
B. Glycogenesis
C. Glycogenolysis
D. Pentose phosphate pathway
Answer: C – Glycogenolysis
Rationale: After 12 hours of fasting, hepatic glycogen stores are mobilized via glycogenolysis
to maintain euglycemia. Glycolysis and glycogenesis are reduced .
2. A 3-day-old neonate presents with poor feeding, vomiting, lethargy, hyperammonemia,
respiratory alkalosis, and low BUN. Which enzyme deficiency is most likely?
A. Ornithine transcarbamylase
B. Fumarase
C. Pyruvate carboxylase
D. Glucose-6-phosphatase
Answer: A – Ornithine transcarbamylase
Rationale: OTC deficiency is an X-linked urea cycle disorder presenting in neonates with
hyperammonemia, respiratory alkalosis, and low BUN .
3. What enzyme catalyzes the rate-limiting step of cholesterol synthesis?
A. HMG-CoA reductase
B. Acetyl-CoA carboxylase
C. Lipoprotein lipase
D. Phosphofructokinase-1
Answer: A – HMG-CoA reductase
Rationale: HMG-CoA reductase is the rate-limiting enzyme in cholesterol synthesis and is
inhibited by statins .
4. A 6-month-old infant presents with developmental delay, coarse facial features, and
hepatosplenomegaly. Enzyme assay reveals alpha-L-iduronidase deficiency. Which
substances accumulate in lysosomes?
A. Glucocerebroside
B. Sphingomyelin
C. Dermatan sulfate and heparan sulfate
D. Galactocerebroside
, 2
Answer: C – Dermatan sulfate and heparan sulfate
Rationale: Alpha-L-iduronidase deficiency causes Hurler syndrome (MPS I), leading to
accumulation of dermatan sulfate and heparan sulfate .
5. A 45-year-old man with chronic alcohol use presents with painful swollen joints and
recurrent kidney stones. Synovial fluid shows needle-shaped, negatively birefringent
crystals. Which pathway is impaired?
A. Heme synthesis
B. Purine salvage
C. Pyrimidine catabolism
D. Urea cycle
Answer: B – Purine salvage
Rationale: Needle-shaped, negatively birefringent crystals indicate gout due to monosodium
urate deposition from impaired purine salvage .
6. Which vitamin deficiency is associated with scurvy?
A. Vitamin A
B. Vitamin C
C. Vitamin D
D. Vitamin K
Answer: B – Vitamin C
Rationale: Vitamin C deficiency leads to impaired collagen synthesis, causing scurvy .
7. A mutation from G to A at position 355 in the beta-globin gene leads to beta-
thalassemia by which mechanism?
A. Nonsense mutation
B. Disruption of normal splicing by creation of a new 3' splice site
C. Frameshift mutation
D. Missense mutation
Answer: B – Disruption of normal splicing
Rationale: This mutation creates a new 3' splice site, disrupting normal splicing and leading
to beta-thalassemia .
8. Which enzyme is deficient in von Gierke disease?
A. Glucose-6-phosphatase
B. Acid alpha-glucosidase
C. Debranching enzyme
D. Muscle phosphorylase
, 3
Answer: A – Glucose-6-phosphatase
Rationale: Von Gierke disease (Type I glycogen storage disease) is caused by glucose-6-
phosphatase deficiency, presenting with hypoglycemia and lactic acidosis .
9. What is the primary function of the pentose phosphate pathway?
A. ATP production
B. NADPH and ribose-5-phosphate generation
C. Glycogen synthesis
D. Fatty acid oxidation
Answer: B – NADPH and ribose-5-phosphate generation
Rationale: The pentose phosphate pathway primarily generates NADPH for reductive
biosynthesis and ribose-5-phosphate for nucleotide synthesis .
10. A newborn has male genital ducts but female external genitalia. Karyotype is 46,XY,
and genetic testing shows a mutation in 5-alpha-reductase. What is the diagnosis?
A. Androgen insensitivity syndrome
B. 5-alpha-reductase deficiency
C. Congenital adrenal hyperplasia
D. Klinefelter syndrome
Answer: B – 5-alpha-reductase deficiency
Rationale: 5-alpha-reductase converts testosterone to DHT. Deficiency causes ambiguous
genitalia with male internal ducts but female external genitalia .
11. Which of the following is a purine salvage pathway enzyme?
A. HGPRT
B. Adenosine deaminase
C. Xanthine oxidase
D. PRPP synthetase
Answer: A – HGPRT
Rationale: Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) is the key enzyme in
purine salvage. Deficiency causes Lesch-Nyhan syndrome .
12. A patient with homocystinuria most likely has a deficiency in which enzyme?
A. Cystathionine beta-synthase
B. Phenylalanine hydroxylase
C. Branched-chain ketoacid dehydrogenase
D. Homogentisate oxidase
NBME CBSE – 100 Practice Questions with Answers
Section 1: Biochemistry & Molecular Biology (Questions 1–15)
1. After 12 hours of fasting, which process is most increased in the liver to maintain blood
glucose?
A. Glycolysis
B. Glycogenesis
C. Glycogenolysis
D. Pentose phosphate pathway
Answer: C – Glycogenolysis
Rationale: After 12 hours of fasting, hepatic glycogen stores are mobilized via glycogenolysis
to maintain euglycemia. Glycolysis and glycogenesis are reduced .
2. A 3-day-old neonate presents with poor feeding, vomiting, lethargy, hyperammonemia,
respiratory alkalosis, and low BUN. Which enzyme deficiency is most likely?
A. Ornithine transcarbamylase
B. Fumarase
C. Pyruvate carboxylase
D. Glucose-6-phosphatase
Answer: A – Ornithine transcarbamylase
Rationale: OTC deficiency is an X-linked urea cycle disorder presenting in neonates with
hyperammonemia, respiratory alkalosis, and low BUN .
3. What enzyme catalyzes the rate-limiting step of cholesterol synthesis?
A. HMG-CoA reductase
B. Acetyl-CoA carboxylase
C. Lipoprotein lipase
D. Phosphofructokinase-1
Answer: A – HMG-CoA reductase
Rationale: HMG-CoA reductase is the rate-limiting enzyme in cholesterol synthesis and is
inhibited by statins .
4. A 6-month-old infant presents with developmental delay, coarse facial features, and
hepatosplenomegaly. Enzyme assay reveals alpha-L-iduronidase deficiency. Which
substances accumulate in lysosomes?
A. Glucocerebroside
B. Sphingomyelin
C. Dermatan sulfate and heparan sulfate
D. Galactocerebroside
, 2
Answer: C – Dermatan sulfate and heparan sulfate
Rationale: Alpha-L-iduronidase deficiency causes Hurler syndrome (MPS I), leading to
accumulation of dermatan sulfate and heparan sulfate .
5. A 45-year-old man with chronic alcohol use presents with painful swollen joints and
recurrent kidney stones. Synovial fluid shows needle-shaped, negatively birefringent
crystals. Which pathway is impaired?
A. Heme synthesis
B. Purine salvage
C. Pyrimidine catabolism
D. Urea cycle
Answer: B – Purine salvage
Rationale: Needle-shaped, negatively birefringent crystals indicate gout due to monosodium
urate deposition from impaired purine salvage .
6. Which vitamin deficiency is associated with scurvy?
A. Vitamin A
B. Vitamin C
C. Vitamin D
D. Vitamin K
Answer: B – Vitamin C
Rationale: Vitamin C deficiency leads to impaired collagen synthesis, causing scurvy .
7. A mutation from G to A at position 355 in the beta-globin gene leads to beta-
thalassemia by which mechanism?
A. Nonsense mutation
B. Disruption of normal splicing by creation of a new 3' splice site
C. Frameshift mutation
D. Missense mutation
Answer: B – Disruption of normal splicing
Rationale: This mutation creates a new 3' splice site, disrupting normal splicing and leading
to beta-thalassemia .
8. Which enzyme is deficient in von Gierke disease?
A. Glucose-6-phosphatase
B. Acid alpha-glucosidase
C. Debranching enzyme
D. Muscle phosphorylase
, 3
Answer: A – Glucose-6-phosphatase
Rationale: Von Gierke disease (Type I glycogen storage disease) is caused by glucose-6-
phosphatase deficiency, presenting with hypoglycemia and lactic acidosis .
9. What is the primary function of the pentose phosphate pathway?
A. ATP production
B. NADPH and ribose-5-phosphate generation
C. Glycogen synthesis
D. Fatty acid oxidation
Answer: B – NADPH and ribose-5-phosphate generation
Rationale: The pentose phosphate pathway primarily generates NADPH for reductive
biosynthesis and ribose-5-phosphate for nucleotide synthesis .
10. A newborn has male genital ducts but female external genitalia. Karyotype is 46,XY,
and genetic testing shows a mutation in 5-alpha-reductase. What is the diagnosis?
A. Androgen insensitivity syndrome
B. 5-alpha-reductase deficiency
C. Congenital adrenal hyperplasia
D. Klinefelter syndrome
Answer: B – 5-alpha-reductase deficiency
Rationale: 5-alpha-reductase converts testosterone to DHT. Deficiency causes ambiguous
genitalia with male internal ducts but female external genitalia .
11. Which of the following is a purine salvage pathway enzyme?
A. HGPRT
B. Adenosine deaminase
C. Xanthine oxidase
D. PRPP synthetase
Answer: A – HGPRT
Rationale: Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) is the key enzyme in
purine salvage. Deficiency causes Lesch-Nyhan syndrome .
12. A patient with homocystinuria most likely has a deficiency in which enzyme?
A. Cystathionine beta-synthase
B. Phenylalanine hydroxylase
C. Branched-chain ketoacid dehydrogenase
D. Homogentisate oxidase
