1
NBME CBSE REAL EXAM 2026/2027 | 200 Questions and
Answers | Complete Exam for USMLE Step 1 | Medical
Examination | Pass Guaranteed - A+ Graded
Section 1: General Principles of Foundational Science
(Biochemistry, Molecular Biology, Genetics) (Questions 1–25)
Q1. A 3-year-old boy is brought to the clinic with developmental delay, fair skin, light
hair, and a musty odor. His plasma phenylalanine level is markedly elevated. Which
enzyme deficiency is responsible for this condition?
A. Homogentisic acid oxidase
B. Phenylalanine hydroxylase
C. Tyrosine kinase
D. Dihydropteridine reductase
E. Cystathionine β-synthase
Correct Answer: B. Phenylalanine hydroxylase [CORRECT]
Rationale: Phenylalanine hydroxylase converts phenylalanine to tyrosine; deficiency
causes phenylketonuria (PKU), presenting with intellectual disability, fair skin, light
hair, and a musty odor. Homogentisic acid oxidase deficiency causes alkaptonuria
(dark urine, ochronosis). Dihydropteridine reductase deficiency causes a rare variant
of PKU. Cystathionine β-synthase deficiency causes homocystinuria (Marfanoid
habitus, lens dislocation). NBME CBSE Content: Amino acid metabolism disorders.
Q2. A 25-year-old man of Ashkenazi Jewish descent develops progressive muscle
weakness and is found to have a deficiency of acid maltase (α-1,4-glucosidase).
Which organelle accumulates glycogen in this disorder?
A. Nucleus
B. Mitochondria
C. Lysosome
,2
D. Peroxisome
E. Smooth endoplasmic reticulum
Correct Answer: C. Lysosome [CORRECT]
Rationale: Pompe disease (type II glycogen storage disease) is caused by lysosomal
acid maltase deficiency, leading to glycogen accumulation in lysosomes. It presents
with cardiomegaly, hypotonia, and hepatomegaly in infants. Von Gierke disease (type
I) involves glucose-6-phosphatase deficiency in the ER. McArdle disease (type V)
involves muscle phosphorylase deficiency in the cytoplasm. NBME CBSE Content:
Glycogen storage diseases.
Q3. A 6-month-old infant presents with hepatosplenomegaly, failure to thrive, and
cherry-red spots on the macula. Enzyme assay reveals deficiency of hexosaminidase
A. Which substrate accumulates in neurons?
A. GM2 ganglioside
B. Glucocerebroside
C. Sphingomyelin
D. Ceramide trihexoside
E. Galactocerebroside
Correct Answer: A. GM2 ganglioside [CORRECT]
Rationale: Tay-Sachs disease (deficiency of hexosaminidase A) causes GM2
ganglioside accumulation in neurons, leading to neurodegeneration and cherry-red
spots. Gaucher disease involves glucocerebroside accumulation. Niemann-Pick
involves sphingomyelin accumulation. Fabry disease involves ceramide trihexoside
accumulation. NBME CBSE Content: Lysosomal storage diseases.
Q4. A 45-year-old man with familial hypercholesterolemia has tendon xanthomas
and a history of premature myocardial infarction. Which protein is defective in this
autosomal dominant disorder?
,3
A. Apolipoprotein B-100
B. LDL receptor
C. HMG-CoA reductase
D. Lipoprotein lipase
E. ABCA1 transporter
Correct Answer: B. LDL receptor [CORRECT]
Rationale: Familial hypercholesterolemia is caused by LDL receptor mutations
(chromosome 19), leading to elevated LDL cholesterol, tendon xanthomas, and
premature atherosclerosis. ApoB-100 defect causes familial defective apoB-100.
ABCA1 deficiency causes Tangier disease (HDL deficiency). NBME CBSE Content: Lipid
metabolism disorders.
Q5. A competitive inhibitor is added to an enzyme reaction. Which kinetic parameter
is unchanged compared to the uninhibited reaction?
A. Vmax
B. Km
C. Vmax/Km ratio
D. Both Vmax and Km
E. Neither Vmax nor Km
Correct Answer: A. Vmax [CORRECT]
Rationale: Competitive inhibitors bind the active site, increasing apparent Km (lower
affinity) but leaving Vmax unchanged if substrate concentration is sufficiently high.
Noncompetitive inhibitors decrease Vmax without changing Km. Uncompetitive
inhibitors decrease both Vmax and Km. NBME CBSE Content: Enzyme kinetics.
Q6. A 2-year-old boy presents with fasting hypoketotic hypoglycemia after a viral
illness. He has elevated C8:0 acylcarnitine on newborn screening. Which enzyme is
deficient?
, 4
A. Glucose-6-phosphatase
B. Medium-chain acyl-CoA dehydrogenase (MCAD)
C. Muscle phosphorylase
D. Carnitine palmitoyltransferase I
E. Very-long-chain acyl-CoA dehydrogenase
Correct Answer: B. Medium-chain acyl-CoA dehydrogenase (MCAD) [CORRECT]
Rationale: MCAD deficiency is the most common fatty acid oxidation disorder,
presenting with fasting hypoketotic hypoglycemia, elevated C8:0 acylcarnitine, and
medium-chain dicarboxylic aciduria. Glucose-6-phosphatase deficiency causes von
Gierke disease (lactic acidosis, hepatomegaly). CPT I deficiency causes carnitine
deficiency with hypoketotic hypoglycemia but different acylcarnitine profile. NBME
CBSE Content: Fatty acid oxidation disorders.
Q7. A 30-year-old man has self-mutilating behavior, choreoathetosis, intellectual
disability, and hyperuricemia. Which enzyme is deficient?
A. Adenosine deaminase
B. Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)
C. Orotidine monophosphate decarboxylase
D. Xanthine oxidase
E. Purine nucleoside phosphorylase
Correct Answer: B. Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)
[CORRECT]
Rationale: Lesch-Nyhan syndrome (HGPRT deficiency) causes hyperuricemia, self-
mutilation, choreoathetosis, and intellectual disability. Adenosine deaminase
deficiency causes SCID. Orotidine monophosphate decarboxylase deficiency causes
orotic aciduria (megaloblastic anemia unresponsive to B12/folate). NBME CBSE
Content: Purine metabolism disorders.
NBME CBSE REAL EXAM 2026/2027 | 200 Questions and
Answers | Complete Exam for USMLE Step 1 | Medical
Examination | Pass Guaranteed - A+ Graded
Section 1: General Principles of Foundational Science
(Biochemistry, Molecular Biology, Genetics) (Questions 1–25)
Q1. A 3-year-old boy is brought to the clinic with developmental delay, fair skin, light
hair, and a musty odor. His plasma phenylalanine level is markedly elevated. Which
enzyme deficiency is responsible for this condition?
A. Homogentisic acid oxidase
B. Phenylalanine hydroxylase
C. Tyrosine kinase
D. Dihydropteridine reductase
E. Cystathionine β-synthase
Correct Answer: B. Phenylalanine hydroxylase [CORRECT]
Rationale: Phenylalanine hydroxylase converts phenylalanine to tyrosine; deficiency
causes phenylketonuria (PKU), presenting with intellectual disability, fair skin, light
hair, and a musty odor. Homogentisic acid oxidase deficiency causes alkaptonuria
(dark urine, ochronosis). Dihydropteridine reductase deficiency causes a rare variant
of PKU. Cystathionine β-synthase deficiency causes homocystinuria (Marfanoid
habitus, lens dislocation). NBME CBSE Content: Amino acid metabolism disorders.
Q2. A 25-year-old man of Ashkenazi Jewish descent develops progressive muscle
weakness and is found to have a deficiency of acid maltase (α-1,4-glucosidase).
Which organelle accumulates glycogen in this disorder?
A. Nucleus
B. Mitochondria
C. Lysosome
,2
D. Peroxisome
E. Smooth endoplasmic reticulum
Correct Answer: C. Lysosome [CORRECT]
Rationale: Pompe disease (type II glycogen storage disease) is caused by lysosomal
acid maltase deficiency, leading to glycogen accumulation in lysosomes. It presents
with cardiomegaly, hypotonia, and hepatomegaly in infants. Von Gierke disease (type
I) involves glucose-6-phosphatase deficiency in the ER. McArdle disease (type V)
involves muscle phosphorylase deficiency in the cytoplasm. NBME CBSE Content:
Glycogen storage diseases.
Q3. A 6-month-old infant presents with hepatosplenomegaly, failure to thrive, and
cherry-red spots on the macula. Enzyme assay reveals deficiency of hexosaminidase
A. Which substrate accumulates in neurons?
A. GM2 ganglioside
B. Glucocerebroside
C. Sphingomyelin
D. Ceramide trihexoside
E. Galactocerebroside
Correct Answer: A. GM2 ganglioside [CORRECT]
Rationale: Tay-Sachs disease (deficiency of hexosaminidase A) causes GM2
ganglioside accumulation in neurons, leading to neurodegeneration and cherry-red
spots. Gaucher disease involves glucocerebroside accumulation. Niemann-Pick
involves sphingomyelin accumulation. Fabry disease involves ceramide trihexoside
accumulation. NBME CBSE Content: Lysosomal storage diseases.
Q4. A 45-year-old man with familial hypercholesterolemia has tendon xanthomas
and a history of premature myocardial infarction. Which protein is defective in this
autosomal dominant disorder?
,3
A. Apolipoprotein B-100
B. LDL receptor
C. HMG-CoA reductase
D. Lipoprotein lipase
E. ABCA1 transporter
Correct Answer: B. LDL receptor [CORRECT]
Rationale: Familial hypercholesterolemia is caused by LDL receptor mutations
(chromosome 19), leading to elevated LDL cholesterol, tendon xanthomas, and
premature atherosclerosis. ApoB-100 defect causes familial defective apoB-100.
ABCA1 deficiency causes Tangier disease (HDL deficiency). NBME CBSE Content: Lipid
metabolism disorders.
Q5. A competitive inhibitor is added to an enzyme reaction. Which kinetic parameter
is unchanged compared to the uninhibited reaction?
A. Vmax
B. Km
C. Vmax/Km ratio
D. Both Vmax and Km
E. Neither Vmax nor Km
Correct Answer: A. Vmax [CORRECT]
Rationale: Competitive inhibitors bind the active site, increasing apparent Km (lower
affinity) but leaving Vmax unchanged if substrate concentration is sufficiently high.
Noncompetitive inhibitors decrease Vmax without changing Km. Uncompetitive
inhibitors decrease both Vmax and Km. NBME CBSE Content: Enzyme kinetics.
Q6. A 2-year-old boy presents with fasting hypoketotic hypoglycemia after a viral
illness. He has elevated C8:0 acylcarnitine on newborn screening. Which enzyme is
deficient?
, 4
A. Glucose-6-phosphatase
B. Medium-chain acyl-CoA dehydrogenase (MCAD)
C. Muscle phosphorylase
D. Carnitine palmitoyltransferase I
E. Very-long-chain acyl-CoA dehydrogenase
Correct Answer: B. Medium-chain acyl-CoA dehydrogenase (MCAD) [CORRECT]
Rationale: MCAD deficiency is the most common fatty acid oxidation disorder,
presenting with fasting hypoketotic hypoglycemia, elevated C8:0 acylcarnitine, and
medium-chain dicarboxylic aciduria. Glucose-6-phosphatase deficiency causes von
Gierke disease (lactic acidosis, hepatomegaly). CPT I deficiency causes carnitine
deficiency with hypoketotic hypoglycemia but different acylcarnitine profile. NBME
CBSE Content: Fatty acid oxidation disorders.
Q7. A 30-year-old man has self-mutilating behavior, choreoathetosis, intellectual
disability, and hyperuricemia. Which enzyme is deficient?
A. Adenosine deaminase
B. Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)
C. Orotidine monophosphate decarboxylase
D. Xanthine oxidase
E. Purine nucleoside phosphorylase
Correct Answer: B. Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)
[CORRECT]
Rationale: Lesch-Nyhan syndrome (HGPRT deficiency) causes hyperuricemia, self-
mutilation, choreoathetosis, and intellectual disability. Adenosine deaminase
deficiency causes SCID. Orotidine monophosphate decarboxylase deficiency causes
orotic aciduria (megaloblastic anemia unresponsive to B12/folate). NBME CBSE
Content: Purine metabolism disorders.
