USMLE Step 1 Exam Practice
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Biochemistry, Genetics & Molecular Biology
Question 1
A 6-month-old infant develops severe fasting hypoglycemia,
lactic acidosis, hyperuricemia, and hepatomegaly. Laboratory
testing demonstrates inability to convert glucose-6-
phosphate to free glucose. Which enzyme is deficient?
A. Phosphofructokinase-1
B. Pyruvate carboxylase
C. Glycogen phosphorylase
D. Glucose-6-phosphatase
E. Branching enzyme
Answer: D. Glucose-6-phosphatase
Glucose-6-phosphatase deficiency causes von Gierke disease.
Glucose-6-phosphate cannot be converted to free glucose in
the liver, kidney, and intestine, causing severe fasting
,hypoglycemia, lactic acidosis, hyperuricemia, hyperlipidemia,
and hepatomegaly.
Question 2
A newborn has hypotonia, seizures, and a characteristic
musty body odor. Serum testing reveals markedly elevated
phenylalanine. Which metabolite is most likely decreased?
A. Tyrosine
B. Homogentisic acid
C. Melanin
D. Dopamine
E. Thyroxine
Answer: A. Tyrosine
Phenylketonuria results from phenylalanine hydroxylase
deficiency or tetrahydrobiopterin deficiency. Phenylalanine
cannot efficiently be converted to tyrosine, making tyrosine
an essential amino acid in affected patients.
Question 3
A child develops episodic vomiting, lethargy, and confusion
after consuming a high-protein meal. Laboratory studies
show hyperammonemia with a low blood urea nitrogen level.
Which enzyme deficiency is most likely?
A. Arginase
B. Ornithine transcarbamylase
C. Argininosuccinate lyase
,D. Carbamoyl phosphate synthetase II
E. Glutamate dehydrogenase
Answer: B. Ornithine transcarbamylase
OTC deficiency is an X-linked urea-cycle disorder
characterized by hyperammonemia and increased urinary
orotic acid. Excess mitochondrial carbamoyl phosphate is
diverted into pyrimidine synthesis.
Question 4
A patient with hereditary fructose intolerance develops
vomiting, diaphoresis, and hypoglycemia after eating fruit.
Which enzyme is deficient?
A. Fructokinase
B. Aldolase B
C. Aldolase A
D. Galactokinase
E. Hexokinase
Answer: B. Aldolase B
Aldolase B deficiency causes hereditary fructose intolerance.
Fructose-1-phosphate accumulates, trapping phosphate and
inhibiting gluconeogenesis and glycogenolysis.
Question 5
A 4-year-old child has recurrent infections, growth
retardation, and neurologic abnormalities. Blood smear
, demonstrates enlarged erythrocytes with nuclear remnants.
Which metabolic disorder is most likely?
A. Glucose-6-phosphate dehydrogenase deficiency
B. Hereditary spherocytosis
C. Lesch-Nyhan syndrome
D. Methylmalonic acidemia
E. Sickle cell disease
Answer: D. Methylmalonic acidemia
Methylmalonic acidemia can result from vitamin B12
deficiency or defects in cobalamin metabolism. It causes
neurologic abnormalities and accumulation of methylmalonic
acid.
Question 6
A researcher inhibits the enzyme responsible for converting
ribonucleotides into deoxyribonucleotides. Which cellular
process is directly impaired?
A. RNA transcription
B. DNA replication
C. Protein translation
D. Glycolysis
E. Fatty acid oxidation
Answer: B. DNA replication
Ribonucleotide reductase converts ribonucleotides to
deoxyribonucleotides, providing substrates required for DNA
synthesis.
Questions And Correct
Answers (Verified Answers)
Plus Rationale 2026 Q&A|
Instant Download Pdf
Biochemistry, Genetics & Molecular Biology
Question 1
A 6-month-old infant develops severe fasting hypoglycemia,
lactic acidosis, hyperuricemia, and hepatomegaly. Laboratory
testing demonstrates inability to convert glucose-6-
phosphate to free glucose. Which enzyme is deficient?
A. Phosphofructokinase-1
B. Pyruvate carboxylase
C. Glycogen phosphorylase
D. Glucose-6-phosphatase
E. Branching enzyme
Answer: D. Glucose-6-phosphatase
Glucose-6-phosphatase deficiency causes von Gierke disease.
Glucose-6-phosphate cannot be converted to free glucose in
the liver, kidney, and intestine, causing severe fasting
,hypoglycemia, lactic acidosis, hyperuricemia, hyperlipidemia,
and hepatomegaly.
Question 2
A newborn has hypotonia, seizures, and a characteristic
musty body odor. Serum testing reveals markedly elevated
phenylalanine. Which metabolite is most likely decreased?
A. Tyrosine
B. Homogentisic acid
C. Melanin
D. Dopamine
E. Thyroxine
Answer: A. Tyrosine
Phenylketonuria results from phenylalanine hydroxylase
deficiency or tetrahydrobiopterin deficiency. Phenylalanine
cannot efficiently be converted to tyrosine, making tyrosine
an essential amino acid in affected patients.
Question 3
A child develops episodic vomiting, lethargy, and confusion
after consuming a high-protein meal. Laboratory studies
show hyperammonemia with a low blood urea nitrogen level.
Which enzyme deficiency is most likely?
A. Arginase
B. Ornithine transcarbamylase
C. Argininosuccinate lyase
,D. Carbamoyl phosphate synthetase II
E. Glutamate dehydrogenase
Answer: B. Ornithine transcarbamylase
OTC deficiency is an X-linked urea-cycle disorder
characterized by hyperammonemia and increased urinary
orotic acid. Excess mitochondrial carbamoyl phosphate is
diverted into pyrimidine synthesis.
Question 4
A patient with hereditary fructose intolerance develops
vomiting, diaphoresis, and hypoglycemia after eating fruit.
Which enzyme is deficient?
A. Fructokinase
B. Aldolase B
C. Aldolase A
D. Galactokinase
E. Hexokinase
Answer: B. Aldolase B
Aldolase B deficiency causes hereditary fructose intolerance.
Fructose-1-phosphate accumulates, trapping phosphate and
inhibiting gluconeogenesis and glycogenolysis.
Question 5
A 4-year-old child has recurrent infections, growth
retardation, and neurologic abnormalities. Blood smear
, demonstrates enlarged erythrocytes with nuclear remnants.
Which metabolic disorder is most likely?
A. Glucose-6-phosphate dehydrogenase deficiency
B. Hereditary spherocytosis
C. Lesch-Nyhan syndrome
D. Methylmalonic acidemia
E. Sickle cell disease
Answer: D. Methylmalonic acidemia
Methylmalonic acidemia can result from vitamin B12
deficiency or defects in cobalamin metabolism. It causes
neurologic abnormalities and accumulation of methylmalonic
acid.
Question 6
A researcher inhibits the enzyme responsible for converting
ribonucleotides into deoxyribonucleotides. Which cellular
process is directly impaired?
A. RNA transcription
B. DNA replication
C. Protein translation
D. Glycolysis
E. Fatty acid oxidation
Answer: B. DNA replication
Ribonucleotide reductase converts ribonucleotides to
deoxyribonucleotides, providing substrates required for DNA
synthesis.
