NBME CBSE Exam – 250 Questions with
Answers & Explanations | Latest 2026 USMLE
Step 1 Prep Guide
NBME CBSE Exam – Latest 2026 prep with 250 multiple-choice questions, verified
answers, and detailed explanations. Covers biochemistry, physiology, pharmacology,
microbiology, immunology, pathology, and biostatistics. Perfect for USMLE Step 1
readiness and guaranteed pass. Instant PDF download.
SECTION 1: BIOCHEMISTRY, MOLECULAR BIOLOGY & GENETICS (Questions 1–40)
1. A 2-year-old child presents with severe developmental delay, cherry-red spot on
macula, and hepatosplenomegaly. Deficiency of which enzyme is most likely?
A. Glucocerebrosidase
B. Hexosaminidase A
C. Sphingomyelinase
D. Alpha-galactosidase A
Answer: B. Tay-Sachs disease is caused by hexosaminidase A deficiency, leading to GM2
ganglioside accumulation. Classic findings include cherry-red macular spot,
neurodegeneration, and early death .
2. A patient with homocystinuria most likely has a defect in which enzyme?
A. Cystathionine beta-synthase
,B. Fumarase
C. Homogentisate oxidase
D. Branched-chain alpha-ketoacid dehydrogenase
Answer: A. Classic homocystinuria results from cystathionine beta-synthase deficiency,
causing elevated homocysteine and methionine. Features include Marfanoid habitus, lens
dislocation, and thrombosis .
3. Which vitamin deficiency causes megaloblastic anemia and neurologic symptoms?
A. Vitamin B1
B. Vitamin B6
C. Vitamin B12
D. Vitamin C
Answer: C. Vitamin B12 (cobalamin) deficiency causes megaloblastic anemia with
hypersegmented neutrophils and neurologic symptoms including subacute combined
degeneration of the spinal cord .
4. A patient with scurvy has defective collagen synthesis due to deficiency of:
A. Vitamin A
B. Vitamin C
C. Vitamin D
D. Vitamin E
Answer: B. Vitamin C (ascorbic acid) is required for hydroxylation of proline and lysine in
collagen synthesis. Deficiency causes scurvy with bleeding gums, poor wound healing, and
perifollicular hemorrhage .
5. Which enzyme is deficient in classic galactosemia?
A. Galactokinase
,B. Galactose-1-phosphate uridyltransferase
C. UDP-galactose epimerase
D. Aldose reductase
Answer: B. Classic galactosemia is caused by galactose-1-phosphate uridyltransferase
(GALT) deficiency. Infants present with failure to thrive, jaundice, and hepatomegaly after
milk ingestion .
6. Which DNA repair defect is associated with xeroderma pigmentosum?
A. Mismatch repair
B. Nucleotide excision repair
C. Base excision repair
D. Nonhomologous end joining
Answer: B. Xeroderma pigmentosum results from defective nucleotide excision repair,
leading to inability to repair UV-induced pyrimidine dimers and extreme photosensitivity with
increased skin cancer risk .
7. A patient with Lesch-Nyhan syndrome has deficiency of:
A. HGPRT
B. APRT
C. ADA
D. Xanthine oxidase
Answer: A. Lesch-Nyhan syndrome is caused by hypoxanthine-guanine
phosphoribosyltransferase (HGPRT) deficiency, leading to hyperuricemia, self-mutilation,
and intellectual disability .
8. Which of the following is the rate-limiting enzyme of glycolysis?
A. Hexokinase
, B. Phosphofructokinase-1
C. Pyruvate kinase
D. Aldolase
Answer: B. Phosphofructokinase-1 (PFK-1) is the rate-limiting enzyme of glycolysis. It is
activated by AMP and fructose-2,6-bisphosphate and inhibited by ATP and citrate .
9. Which molecule is the major source of energy for the brain during prolonged
fasting?
A. Glucose
B. Ketone bodies
C. Fatty acids
D. Amino acids
Answer: B. During prolonged fasting, the brain adapts to use ketone bodies (beta-
hydroxybutyrate and acetoacetate) as its primary energy source, sparing glucose for red
blood cells .
10. A patient with orotic aciduria improves with uridine supplementation. The
defective enzyme is:
A. Carbamoyl phosphate synthetase II
B. Aspartate transcarbamoylase
C. Dihydroorotase
D. UMP synthase
Answer: D. Orotic aciduria results from UMP synthase deficiency, impairing pyrimidine
synthesis. Uridine supplementation bypasses the defect and provides pyrimidines .
11. A newborn develops jaundice, hepatomegaly, and hypoglycemia after fasting.
Liver biopsy reveals increased glycogen with abnormal structure. Which enzyme is
Answers & Explanations | Latest 2026 USMLE
Step 1 Prep Guide
NBME CBSE Exam – Latest 2026 prep with 250 multiple-choice questions, verified
answers, and detailed explanations. Covers biochemistry, physiology, pharmacology,
microbiology, immunology, pathology, and biostatistics. Perfect for USMLE Step 1
readiness and guaranteed pass. Instant PDF download.
SECTION 1: BIOCHEMISTRY, MOLECULAR BIOLOGY & GENETICS (Questions 1–40)
1. A 2-year-old child presents with severe developmental delay, cherry-red spot on
macula, and hepatosplenomegaly. Deficiency of which enzyme is most likely?
A. Glucocerebrosidase
B. Hexosaminidase A
C. Sphingomyelinase
D. Alpha-galactosidase A
Answer: B. Tay-Sachs disease is caused by hexosaminidase A deficiency, leading to GM2
ganglioside accumulation. Classic findings include cherry-red macular spot,
neurodegeneration, and early death .
2. A patient with homocystinuria most likely has a defect in which enzyme?
A. Cystathionine beta-synthase
,B. Fumarase
C. Homogentisate oxidase
D. Branched-chain alpha-ketoacid dehydrogenase
Answer: A. Classic homocystinuria results from cystathionine beta-synthase deficiency,
causing elevated homocysteine and methionine. Features include Marfanoid habitus, lens
dislocation, and thrombosis .
3. Which vitamin deficiency causes megaloblastic anemia and neurologic symptoms?
A. Vitamin B1
B. Vitamin B6
C. Vitamin B12
D. Vitamin C
Answer: C. Vitamin B12 (cobalamin) deficiency causes megaloblastic anemia with
hypersegmented neutrophils and neurologic symptoms including subacute combined
degeneration of the spinal cord .
4. A patient with scurvy has defective collagen synthesis due to deficiency of:
A. Vitamin A
B. Vitamin C
C. Vitamin D
D. Vitamin E
Answer: B. Vitamin C (ascorbic acid) is required for hydroxylation of proline and lysine in
collagen synthesis. Deficiency causes scurvy with bleeding gums, poor wound healing, and
perifollicular hemorrhage .
5. Which enzyme is deficient in classic galactosemia?
A. Galactokinase
,B. Galactose-1-phosphate uridyltransferase
C. UDP-galactose epimerase
D. Aldose reductase
Answer: B. Classic galactosemia is caused by galactose-1-phosphate uridyltransferase
(GALT) deficiency. Infants present with failure to thrive, jaundice, and hepatomegaly after
milk ingestion .
6. Which DNA repair defect is associated with xeroderma pigmentosum?
A. Mismatch repair
B. Nucleotide excision repair
C. Base excision repair
D. Nonhomologous end joining
Answer: B. Xeroderma pigmentosum results from defective nucleotide excision repair,
leading to inability to repair UV-induced pyrimidine dimers and extreme photosensitivity with
increased skin cancer risk .
7. A patient with Lesch-Nyhan syndrome has deficiency of:
A. HGPRT
B. APRT
C. ADA
D. Xanthine oxidase
Answer: A. Lesch-Nyhan syndrome is caused by hypoxanthine-guanine
phosphoribosyltransferase (HGPRT) deficiency, leading to hyperuricemia, self-mutilation,
and intellectual disability .
8. Which of the following is the rate-limiting enzyme of glycolysis?
A. Hexokinase
, B. Phosphofructokinase-1
C. Pyruvate kinase
D. Aldolase
Answer: B. Phosphofructokinase-1 (PFK-1) is the rate-limiting enzyme of glycolysis. It is
activated by AMP and fructose-2,6-bisphosphate and inhibited by ATP and citrate .
9. Which molecule is the major source of energy for the brain during prolonged
fasting?
A. Glucose
B. Ketone bodies
C. Fatty acids
D. Amino acids
Answer: B. During prolonged fasting, the brain adapts to use ketone bodies (beta-
hydroxybutyrate and acetoacetate) as its primary energy source, sparing glucose for red
blood cells .
10. A patient with orotic aciduria improves with uridine supplementation. The
defective enzyme is:
A. Carbamoyl phosphate synthetase II
B. Aspartate transcarbamoylase
C. Dihydroorotase
D. UMP synthase
Answer: D. Orotic aciduria results from UMP synthase deficiency, impairing pyrimidine
synthesis. Uridine supplementation bypasses the defect and provides pyrimidines .
11. A newborn develops jaundice, hepatomegaly, and hypoglycemia after fasting.
Liver biopsy reveals increased glycogen with abnormal structure. Which enzyme is
