NBME Biochemistry Shelf Exam Actual
Exam 2026/2027 with Detailed Rationales
| Complete Exam-Style Questions | Pass
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Q1: A 3-year-old male presents with acute hemolytic anemia after consuming fava beans.
Peripheral blood smear shows bite cells and Heinz bodies. Which enzyme deficiency is
most likely responsible for this patient's condition?
A. Pyruvate kinase
B. Glucose-6-phosphate dehydrogenase
C. Glucose-6-phosphate dehydrogenase [CORRECT]
D. Glutathione synthase
Correct Answer: C
Rationale: Correct because glucose-6-phosphate dehydrogenase (G6PD) deficiency
impairs the pentose phosphate pathway, reducing NADPH production and leading to
oxidative damage in red blood cells when exposed to fava beans, causing hemolysis.
Q2: Which of the following amino acids is classified as a basic amino acid at physiological
pH?
A. Alanine
B. Aspartic acid
,C. Lysine [CORRECT]
D. Serine
Correct Answer: C
Rationale: Correct because lysine has a positively charged side chain at physiological pH,
making it a basic amino acid, unlike alanine (nonpolar), aspartic acid (acidic), or serine
(polar uncharged).
Q3: A Lineweaver-Burk plot of an enzyme-catalyzed reaction shows a family of lines
intersecting on the y-axis. Which type of inhibition is most likely present?
A. Competitive inhibition
B. Uncompetitive inhibition
C. Noncompetitive inhibition [CORRECT]
D. Mixed inhibition
Correct Answer: C
Rationale: Correct because noncompetitive inhibition results in lines intersecting on the
y-axis (1/Vmax) in a Lineweaver-Burk plot, as the inhibitor binds to a site other than the
active site, affecting Vmax but not Km.
Q4: Which enzyme is responsible for the rate-limiting step in glycolysis and is allosterically
activated by fructose-2,6-bisphosphate?
A. Hexokinase
B. Glucokinase
C. Phosphofructokinase-1 [CORRECT]
D. Pyruvate kinase
Correct Answer: C
Rationale: Correct because phosphofructokinase-1 (PFK-1) is the rate-limiting enzyme in
glycolysis, and its activity is enhanced by fructose-2,6-bisphosphate, a key regulator of
glycolytic flux.
,Q5: A couple has a child with an autosomal recessive disorder. Neither parent is affected.
What is the probability that their next child will be a carrier of the disorder?
A. 25%
B. 50%
C. 50% [CORRECT]
D. 75%
Correct Answer: C
Rationale: Correct because for an autosomal recessive disorder, both parents must be
carriers (heterozygous). The probability of their next child being a carrier (heterozygous) is
50%, as each parent has a 50% chance of passing the mutant allele.
Q6: Which vitamin is required as a cofactor for the hydroxylation of proline and lysine
residues in collagen synthesis?
A. Vitamin B1 (Thiamine)
B. Vitamin B3 (Niacin)
C. Vitamin C (Ascorbic acid) [CORRECT]
D. Vitamin K
Correct Answer: C
Rationale: Correct because vitamin C (ascorbic acid) is essential for the hydroxylation of
proline and lysine in collagen, a process critical for collagen stability, and its deficiency
leads to scurvy.
Q7: Which of the following enzymes is responsible for synthesizing the leading strand
during DNA replication in eukaryotes?
A. DNA polymerase I
B. DNA polymerase III
C. DNA polymerase delta/epsilon [CORRECT]
D. DNA ligase
, Correct Answer: C
Rationale: Correct because in eukaryotes, DNA polymerase epsilon is primarily
responsible for leading strand synthesis, while delta synthesizes the lagging strand during
DNA replication.
Q8: Which lipoprotein is primarily responsible for transporting dietary triglycerides from the
intestines to peripheral tissues?
A. VLDL
B. LDL
C. Chylomicrons [CORRECT]
D. HDL
Correct Answer: C
Rationale: Correct because chylomicrons are synthesized in the intestines and transport
dietary triglycerides and cholesterol to peripheral tissues via the lymphatic system and
bloodstream.
Q9: A newborn presents with hyperammonemia, lethargy, and vomiting. Laboratory tests
reveal elevated orotic acid in the urine. Which urea cycle enzyme is most likely deficient?
A. Carbamoyl phosphate synthetase I
B. Ornithine transcarbamylase
C. Ornithine transcarbamylase [CORRECT]
D. Argininosuccinate synthetase
Correct Answer: C
Rationale: Correct because ornithine transcarbamylase (OTC) deficiency leads to
accumulation of carbamoyl phosphate, which is diverted to pyrimidine synthesis, causing
orotic aciduria, a hallmark of OTC deficiency.
Exam 2026/2027 with Detailed Rationales
| Complete Exam-Style Questions | Pass
Guaranteed – A+ Graded
Q1: A 3-year-old male presents with acute hemolytic anemia after consuming fava beans.
Peripheral blood smear shows bite cells and Heinz bodies. Which enzyme deficiency is
most likely responsible for this patient's condition?
A. Pyruvate kinase
B. Glucose-6-phosphate dehydrogenase
C. Glucose-6-phosphate dehydrogenase [CORRECT]
D. Glutathione synthase
Correct Answer: C
Rationale: Correct because glucose-6-phosphate dehydrogenase (G6PD) deficiency
impairs the pentose phosphate pathway, reducing NADPH production and leading to
oxidative damage in red blood cells when exposed to fava beans, causing hemolysis.
Q2: Which of the following amino acids is classified as a basic amino acid at physiological
pH?
A. Alanine
B. Aspartic acid
,C. Lysine [CORRECT]
D. Serine
Correct Answer: C
Rationale: Correct because lysine has a positively charged side chain at physiological pH,
making it a basic amino acid, unlike alanine (nonpolar), aspartic acid (acidic), or serine
(polar uncharged).
Q3: A Lineweaver-Burk plot of an enzyme-catalyzed reaction shows a family of lines
intersecting on the y-axis. Which type of inhibition is most likely present?
A. Competitive inhibition
B. Uncompetitive inhibition
C. Noncompetitive inhibition [CORRECT]
D. Mixed inhibition
Correct Answer: C
Rationale: Correct because noncompetitive inhibition results in lines intersecting on the
y-axis (1/Vmax) in a Lineweaver-Burk plot, as the inhibitor binds to a site other than the
active site, affecting Vmax but not Km.
Q4: Which enzyme is responsible for the rate-limiting step in glycolysis and is allosterically
activated by fructose-2,6-bisphosphate?
A. Hexokinase
B. Glucokinase
C. Phosphofructokinase-1 [CORRECT]
D. Pyruvate kinase
Correct Answer: C
Rationale: Correct because phosphofructokinase-1 (PFK-1) is the rate-limiting enzyme in
glycolysis, and its activity is enhanced by fructose-2,6-bisphosphate, a key regulator of
glycolytic flux.
,Q5: A couple has a child with an autosomal recessive disorder. Neither parent is affected.
What is the probability that their next child will be a carrier of the disorder?
A. 25%
B. 50%
C. 50% [CORRECT]
D. 75%
Correct Answer: C
Rationale: Correct because for an autosomal recessive disorder, both parents must be
carriers (heterozygous). The probability of their next child being a carrier (heterozygous) is
50%, as each parent has a 50% chance of passing the mutant allele.
Q6: Which vitamin is required as a cofactor for the hydroxylation of proline and lysine
residues in collagen synthesis?
A. Vitamin B1 (Thiamine)
B. Vitamin B3 (Niacin)
C. Vitamin C (Ascorbic acid) [CORRECT]
D. Vitamin K
Correct Answer: C
Rationale: Correct because vitamin C (ascorbic acid) is essential for the hydroxylation of
proline and lysine in collagen, a process critical for collagen stability, and its deficiency
leads to scurvy.
Q7: Which of the following enzymes is responsible for synthesizing the leading strand
during DNA replication in eukaryotes?
A. DNA polymerase I
B. DNA polymerase III
C. DNA polymerase delta/epsilon [CORRECT]
D. DNA ligase
, Correct Answer: C
Rationale: Correct because in eukaryotes, DNA polymerase epsilon is primarily
responsible for leading strand synthesis, while delta synthesizes the lagging strand during
DNA replication.
Q8: Which lipoprotein is primarily responsible for transporting dietary triglycerides from the
intestines to peripheral tissues?
A. VLDL
B. LDL
C. Chylomicrons [CORRECT]
D. HDL
Correct Answer: C
Rationale: Correct because chylomicrons are synthesized in the intestines and transport
dietary triglycerides and cholesterol to peripheral tissues via the lymphatic system and
bloodstream.
Q9: A newborn presents with hyperammonemia, lethargy, and vomiting. Laboratory tests
reveal elevated orotic acid in the urine. Which urea cycle enzyme is most likely deficient?
A. Carbamoyl phosphate synthetase I
B. Ornithine transcarbamylase
C. Ornithine transcarbamylase [CORRECT]
D. Argininosuccinate synthetase
Correct Answer: C
Rationale: Correct because ornithine transcarbamylase (OTC) deficiency leads to
accumulation of carbamoyl phosphate, which is diverted to pyrimidine synthesis, causing
orotic aciduria, a hallmark of OTC deficiency.
