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NBME CBSE 2026/2027 — 100 Practice Questions with Answers (English)
Section 1: Biochemistry & Molecular Biology (Questions 1–15)
1. A 4-month-old infant presents with failure to thrive, hepatomegaly, and hypoglycemia
after overnight fasting. Liver biopsy shows increased glycogen with abnormal structure.
Which enzyme is most likely deficient?
A) Glucose-6-phosphatase
B) Debranching enzyme
C) Branching enzyme
D) Muscle phosphorylase
Answer: C
Branching enzyme (α-1,4 → α-1,6) deficiency causes Andersen disease (GSD IV),
characterized by abnormal glycogen structure and cirrhosis .
2. A 2-year-old child presents with fasting hypoglycemia, hepatomegaly, and elevated
lactate. Genetic testing confirms glucose-6-phosphatase deficiency. What is the diagnosis?
A) Von Gierke disease (GSD I)
B) Pompe disease (GSD II)
C) Cori disease (GSD III)
D) McArdle disease (GSD V)
Answer: A
Glucose-6-phosphatase deficiency (GSD I) prevents the final step of gluconeogenesis and
glycogenolysis, causing severe fasting hypoglycemia, hepatomegaly, and lactic acidosis .
3. A 6-month-old infant presents with cardiomegaly, hypotonia, and macroglossia. He dies
from cardiac failure. Autopsy shows massive glycogen accumulation in lysosomes. Which
enzyme is deficient?
A) Acid α-glucosidase (lysosomal)
B) Muscle phosphorylase
C) Debranching enzyme
D) Branching enzyme
Answer: A
Pompe disease (GSD II) results from lysosomal acid α-glucosidase deficiency, causing
glycogen accumulation in lysosomes, particularly in cardiac and skeletal muscle .
4. A 25-year-old man experiences exercise intolerance, myalgia, and myoglobinuria after
intense exercise. Symptoms improve with rest. Which enzyme is deficient?
A) Muscle phosphorylase (myophosphorylase)
B) Hepatic phosphorylase
C) Debranching enzyme
D) Glucose-6-phosphatase
, 2
Answer: A
McArdle disease (GSD V) is caused by muscle phosphorylase deficiency, presenting with
exercise intolerance, cramps, and myoglobinuria .
5. A 3-day-old neonate develops vomiting, lethargy, and hyperammonemia after feeding.
Plasma citrulline is low, and urine orotic acid is elevated. Which enzyme is deficient?
A) Carbamoyl phosphate synthetase I
B) Ornithine transcarbamylase
C) Argininosuccinate synthetase
D) Arginase
Answer: B
Ornithine transcarbamylase (OTC) deficiency, an X-linked urea cycle disorder, causes
hyperammonemia with low citrulline and elevated orotic acid .
6. A 6-month-old infant presents with fair skin, blue eyes, and recurrent infections. He has
a musty odor. Which enzyme is deficient?
A) Homogentisate oxidase
B) Phenylalanine hydroxylase
C) Branched-chain α-ketoacid dehydrogenase
D) Cystathionine β-synthase
Answer: B
Phenylketonuria (PKU) results from phenylalanine hydroxylase deficiency, causing
phenylalanine accumulation, musty odor, fair skin, and intellectual disability if untreated .
7. After 12 hours of fasting, which process is most increased in the liver to maintain blood
glucose levels?
A) Glycolysis
B) Glycogenesis
C) Glycogenolysis
D) Pentose phosphate pathway
Answer: C
After 12 hours of fasting, hepatic glycogen stores are mobilized via glycogenolysis to
maintain euglycemia .
8. A 6-month-old infant presents with developmental delay, coarse facial features, and
hepatosplenomegaly. Enzyme assay reveals deficiency of alpha-L-iduronidase. Which
substances accumulate in lysosomes?
A) Glucocerebroside
B) Sphingomyelin
C) Dermatan sulfate and heparan sulfate
D) Galactocerebroside
, 3
Answer: C
Alpha-L-iduronidase deficiency causes Hurler syndrome (mucopolysaccharidosis I), leading
to accumulation of dermatan sulfate and heparan sulfate .
9. 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 metabolic pathway is impaired?
A) Heme synthesis
B) Purine salvage
C) Pyrimidine catabolism
D) Urea cycle
Answer: B
Needle-shaped, negatively birefringent crystals indicate gout due to monosodium urate
deposition from impaired purine salvage .
10. A 5-year-old boy has recurrent bacterial infections, poor wound healing, and bleeding
tendency. Platelet count is normal, but bleeding time is prolonged. Which protein is
deficient?
A) Factor VIII
B) Glycoprotein IIb/IIIa
C) Platelet-derived growth factor (PDGF)
D) Integrin αIIbβ3
Answer: D
Glanzmann thrombasthenia is caused by deficiency of integrin αIIbβ3 (GPIIb/IIIa) receptor
on platelets, essential for platelet aggregation .
11. A 45-year-old man with hypertension and diabetes is prescribed a medication that
inhibits HMG-CoA reductase. What is the primary effect?
A) Increased VLDL synthesis
B) Decreased LDL receptor expression
C) Decreased de novo cholesterol synthesis
D) Increased bile acid synthesis
Answer: C
HMG-CoA reductase is the rate-limiting enzyme in cholesterol synthesis. Inhibiting it directly
reduces cholesterol production .
12. A 2-month-old infant presents with failure to thrive, vomiting, and lethargy. Labs show
severe metabolic acidosis with elevated anion gap and increased methylmalonic acid.
Which vitamin deficiency is contributing?
A) Vitamin B12
B) Vitamin B6
NBME CBSE 2026/2027 — 100 Practice Questions with Answers (English)
Section 1: Biochemistry & Molecular Biology (Questions 1–15)
1. A 4-month-old infant presents with failure to thrive, hepatomegaly, and hypoglycemia
after overnight fasting. Liver biopsy shows increased glycogen with abnormal structure.
Which enzyme is most likely deficient?
A) Glucose-6-phosphatase
B) Debranching enzyme
C) Branching enzyme
D) Muscle phosphorylase
Answer: C
Branching enzyme (α-1,4 → α-1,6) deficiency causes Andersen disease (GSD IV),
characterized by abnormal glycogen structure and cirrhosis .
2. A 2-year-old child presents with fasting hypoglycemia, hepatomegaly, and elevated
lactate. Genetic testing confirms glucose-6-phosphatase deficiency. What is the diagnosis?
A) Von Gierke disease (GSD I)
B) Pompe disease (GSD II)
C) Cori disease (GSD III)
D) McArdle disease (GSD V)
Answer: A
Glucose-6-phosphatase deficiency (GSD I) prevents the final step of gluconeogenesis and
glycogenolysis, causing severe fasting hypoglycemia, hepatomegaly, and lactic acidosis .
3. A 6-month-old infant presents with cardiomegaly, hypotonia, and macroglossia. He dies
from cardiac failure. Autopsy shows massive glycogen accumulation in lysosomes. Which
enzyme is deficient?
A) Acid α-glucosidase (lysosomal)
B) Muscle phosphorylase
C) Debranching enzyme
D) Branching enzyme
Answer: A
Pompe disease (GSD II) results from lysosomal acid α-glucosidase deficiency, causing
glycogen accumulation in lysosomes, particularly in cardiac and skeletal muscle .
4. A 25-year-old man experiences exercise intolerance, myalgia, and myoglobinuria after
intense exercise. Symptoms improve with rest. Which enzyme is deficient?
A) Muscle phosphorylase (myophosphorylase)
B) Hepatic phosphorylase
C) Debranching enzyme
D) Glucose-6-phosphatase
, 2
Answer: A
McArdle disease (GSD V) is caused by muscle phosphorylase deficiency, presenting with
exercise intolerance, cramps, and myoglobinuria .
5. A 3-day-old neonate develops vomiting, lethargy, and hyperammonemia after feeding.
Plasma citrulline is low, and urine orotic acid is elevated. Which enzyme is deficient?
A) Carbamoyl phosphate synthetase I
B) Ornithine transcarbamylase
C) Argininosuccinate synthetase
D) Arginase
Answer: B
Ornithine transcarbamylase (OTC) deficiency, an X-linked urea cycle disorder, causes
hyperammonemia with low citrulline and elevated orotic acid .
6. A 6-month-old infant presents with fair skin, blue eyes, and recurrent infections. He has
a musty odor. Which enzyme is deficient?
A) Homogentisate oxidase
B) Phenylalanine hydroxylase
C) Branched-chain α-ketoacid dehydrogenase
D) Cystathionine β-synthase
Answer: B
Phenylketonuria (PKU) results from phenylalanine hydroxylase deficiency, causing
phenylalanine accumulation, musty odor, fair skin, and intellectual disability if untreated .
7. After 12 hours of fasting, which process is most increased in the liver to maintain blood
glucose levels?
A) Glycolysis
B) Glycogenesis
C) Glycogenolysis
D) Pentose phosphate pathway
Answer: C
After 12 hours of fasting, hepatic glycogen stores are mobilized via glycogenolysis to
maintain euglycemia .
8. A 6-month-old infant presents with developmental delay, coarse facial features, and
hepatosplenomegaly. Enzyme assay reveals deficiency of alpha-L-iduronidase. Which
substances accumulate in lysosomes?
A) Glucocerebroside
B) Sphingomyelin
C) Dermatan sulfate and heparan sulfate
D) Galactocerebroside
, 3
Answer: C
Alpha-L-iduronidase deficiency causes Hurler syndrome (mucopolysaccharidosis I), leading
to accumulation of dermatan sulfate and heparan sulfate .
9. 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 metabolic pathway is impaired?
A) Heme synthesis
B) Purine salvage
C) Pyrimidine catabolism
D) Urea cycle
Answer: B
Needle-shaped, negatively birefringent crystals indicate gout due to monosodium urate
deposition from impaired purine salvage .
10. A 5-year-old boy has recurrent bacterial infections, poor wound healing, and bleeding
tendency. Platelet count is normal, but bleeding time is prolonged. Which protein is
deficient?
A) Factor VIII
B) Glycoprotein IIb/IIIa
C) Platelet-derived growth factor (PDGF)
D) Integrin αIIbβ3
Answer: D
Glanzmann thrombasthenia is caused by deficiency of integrin αIIbβ3 (GPIIb/IIIa) receptor
on platelets, essential for platelet aggregation .
11. A 45-year-old man with hypertension and diabetes is prescribed a medication that
inhibits HMG-CoA reductase. What is the primary effect?
A) Increased VLDL synthesis
B) Decreased LDL receptor expression
C) Decreased de novo cholesterol synthesis
D) Increased bile acid synthesis
Answer: C
HMG-CoA reductase is the rate-limiting enzyme in cholesterol synthesis. Inhibiting it directly
reduces cholesterol production .
12. A 2-month-old infant presents with failure to thrive, vomiting, and lethargy. Labs show
severe metabolic acidosis with elevated anion gap and increased methylmalonic acid.
Which vitamin deficiency is contributing?
A) Vitamin B12
B) Vitamin B6
