USMLE Step 1 Practice Test Bank Verified
Questions, Correct Answers, and Detailed
Explanations for Computer Science
Students||Already Graded A+
1.
A 6-year-old child presents with recurrent infections and absent thymic
shadow on chest X-ray. Serum calcium is low. Which embryologic defect is
most likely responsible?
A. Failure of neural crest migration
B. Failure of mesoderm differentiation
C. Failure of endodermal pouch development
D. Failure of ectodermal placode formation
E. Failure of somite segmentation
Answer: C
DiGeorge syndrome results from failure of development of the 3rd and 4th
pharyngeal pouches (endoderm), leading to thymic aplasia, hypocalcemia, and
recurrent infections.
2.
A drug that competitively inhibits angiotensin-converting enzyme would most
directly decrease which of the following?
A. Bradykinin breakdown
B. Aldosterone secretion
C. Renin release
D. Sodium delivery to macula densa
E. Sympathetic outflow
Answer: B
ACE inhibitors reduce angiotensin II formation, which directly decreases
aldosterone secretion from the adrenal cortex.
,3.
A patient has a mutation causing inability to repair thymine dimers. This
patient is at highest risk for which condition?
A. Acute lymphoblastic leukemia
B. Xeroderma pigmentosum
C. Ataxia-telangiectasia
D. Fanconi anemia
E. Bloom syndrome
Answer: B
Xeroderma pigmentosum results from defective nucleotide excision repair,
leading to accumulation of thymine dimers and extreme UV sensitivity.
4.
Which enzyme is responsible for the rate-limiting step of glycolysis?
A. Hexokinase
B. Phosphoglucose isomerase
C. Phosphofructokinase-1
D. Pyruvate kinase
E. Lactate dehydrogenase
Answer: C
Phosphofructokinase-1 (PFK-1) is the rate-limiting and most highly regulated
enzyme of glycolysis.
5.
A newborn presents with hypotonia, poor feeding, and seizures. Lab tests
reveal elevated ammonia levels. Which enzyme deficiency is most likely?
A. Arginase
B. Carbamoyl phosphate synthetase I
C. Phenylalanine hydroxylase
,D. Tyrosinase
E. Branched-chain α-ketoacid dehydrogenase
Answer: B
Carbamoyl phosphate synthetase I deficiency causes hyperammonemia due to
impaired urea cycle function.
6.
Which immunoglobulin is primarily responsible for mucosal immunity?
A. IgA
B. IgE
C. IgG
D. IgM
E. IgD
Answer: A
IgA is secreted in mucosal surfaces, breast milk, saliva, and tears and protects
against pathogens at epithelial surfaces.
7.
A patient with myasthenia gravis is treated with a drug that increases
acetylcholine at the neuromuscular junction. What is the mechanism?
A. Blocking sodium channels
B. Inhibiting acetylcholinesterase
C. Blocking calcium channels
D. Inhibiting choline uptake
E. Activating muscarinic receptors
Answer: B
Acetylcholinesterase inhibitors increase acetylcholine availability at the
neuromuscular junction, improving muscle strength.
8.
, Which vitamin deficiency leads to impaired γ-carboxylation of clotting factors?
A. Vitamin A
B. Vitamin B12
C. Vitamin C
D. Vitamin D
E. Vitamin K
Answer: E
Vitamin K is required for γ-carboxylation of clotting factors II, VII, IX, and X.
9.
A mutation in which gene is most likely responsible for chronic myelogenous
leukemia?
A. BCL2
B. MYC
C. TP53
D. BCR-ABL
E. RAS
Answer: D
CML is caused by the Philadelphia chromosome t(9;22), producing the BCR-ABL
fusion protein with constitutive tyrosine kinase activity.
10.
Which neurotransmitter is primarily inhibitory in the central nervous system?
A. Dopamine
B. Glutamate
C. Acetylcholine
D. GABA
E. Norepinephrine
Answer: D
GABA is the main inhibitory neurotransmitter in the CNS.
Questions, Correct Answers, and Detailed
Explanations for Computer Science
Students||Already Graded A+
1.
A 6-year-old child presents with recurrent infections and absent thymic
shadow on chest X-ray. Serum calcium is low. Which embryologic defect is
most likely responsible?
A. Failure of neural crest migration
B. Failure of mesoderm differentiation
C. Failure of endodermal pouch development
D. Failure of ectodermal placode formation
E. Failure of somite segmentation
Answer: C
DiGeorge syndrome results from failure of development of the 3rd and 4th
pharyngeal pouches (endoderm), leading to thymic aplasia, hypocalcemia, and
recurrent infections.
2.
A drug that competitively inhibits angiotensin-converting enzyme would most
directly decrease which of the following?
A. Bradykinin breakdown
B. Aldosterone secretion
C. Renin release
D. Sodium delivery to macula densa
E. Sympathetic outflow
Answer: B
ACE inhibitors reduce angiotensin II formation, which directly decreases
aldosterone secretion from the adrenal cortex.
,3.
A patient has a mutation causing inability to repair thymine dimers. This
patient is at highest risk for which condition?
A. Acute lymphoblastic leukemia
B. Xeroderma pigmentosum
C. Ataxia-telangiectasia
D. Fanconi anemia
E. Bloom syndrome
Answer: B
Xeroderma pigmentosum results from defective nucleotide excision repair,
leading to accumulation of thymine dimers and extreme UV sensitivity.
4.
Which enzyme is responsible for the rate-limiting step of glycolysis?
A. Hexokinase
B. Phosphoglucose isomerase
C. Phosphofructokinase-1
D. Pyruvate kinase
E. Lactate dehydrogenase
Answer: C
Phosphofructokinase-1 (PFK-1) is the rate-limiting and most highly regulated
enzyme of glycolysis.
5.
A newborn presents with hypotonia, poor feeding, and seizures. Lab tests
reveal elevated ammonia levels. Which enzyme deficiency is most likely?
A. Arginase
B. Carbamoyl phosphate synthetase I
C. Phenylalanine hydroxylase
,D. Tyrosinase
E. Branched-chain α-ketoacid dehydrogenase
Answer: B
Carbamoyl phosphate synthetase I deficiency causes hyperammonemia due to
impaired urea cycle function.
6.
Which immunoglobulin is primarily responsible for mucosal immunity?
A. IgA
B. IgE
C. IgG
D. IgM
E. IgD
Answer: A
IgA is secreted in mucosal surfaces, breast milk, saliva, and tears and protects
against pathogens at epithelial surfaces.
7.
A patient with myasthenia gravis is treated with a drug that increases
acetylcholine at the neuromuscular junction. What is the mechanism?
A. Blocking sodium channels
B. Inhibiting acetylcholinesterase
C. Blocking calcium channels
D. Inhibiting choline uptake
E. Activating muscarinic receptors
Answer: B
Acetylcholinesterase inhibitors increase acetylcholine availability at the
neuromuscular junction, improving muscle strength.
8.
, Which vitamin deficiency leads to impaired γ-carboxylation of clotting factors?
A. Vitamin A
B. Vitamin B12
C. Vitamin C
D. Vitamin D
E. Vitamin K
Answer: E
Vitamin K is required for γ-carboxylation of clotting factors II, VII, IX, and X.
9.
A mutation in which gene is most likely responsible for chronic myelogenous
leukemia?
A. BCL2
B. MYC
C. TP53
D. BCR-ABL
E. RAS
Answer: D
CML is caused by the Philadelphia chromosome t(9;22), producing the BCR-ABL
fusion protein with constitutive tyrosine kinase activity.
10.
Which neurotransmitter is primarily inhibitory in the central nervous system?
A. Dopamine
B. Glutamate
C. Acetylcholine
D. GABA
E. Norepinephrine
Answer: D
GABA is the main inhibitory neurotransmitter in the CNS.
