USMLE Step 1 Test Bank Examination
Newest With Complete Questions And
Correct Detailed Answers| Brand New
Version!2026 2027
1. A 6-year-old boy presents with recurrent infections and
eczema. Laboratory studies show elevated IgE levels. Which
cytokine is most responsible for IgE class switching?
A. IFN-γ
B. IL-2
C. IL-4
D. IL-12
Answer: C. IL-4
Bold italics: IL-4 promotes B-cell class switching to IgE and
IgG4 and is produced by Th2 cells.
2. A mutation in the dystrophin gene most directly affects
which cellular structure?
A. Sarcomere
B. Z-line
C. Costamere
D. Mitochondria
,Answer: C. Costamere
Bold italics: Dystrophin anchors muscle fibers to the
extracellular matrix via costameres.
3. Which enzyme is inhibited by statins?
A. HMG-CoA synthase
B. HMG-CoA reductase
C. Acetyl-CoA carboxylase
D. LDL receptor
Answer: B. HMG-CoA reductase
Bold italics: Statins inhibit the rate-limiting enzyme in
cholesterol synthesis.
4. A patient with chronic alcoholism develops confusion,
ophthalmoplegia, and ataxia. Which vitamin deficiency is
responsible?
A. Vitamin B6
B. Vitamin B12
C. Thiamine
D. Niacin
Answer: C. Thiamine
Bold italics: Thiamine deficiency causes Wernicke
encephalopathy.
,5. Which acid–base disturbance is expected in a patient with
prolonged vomiting?
A. Metabolic acidosis
B. Respiratory acidosis
C. Metabolic alkalosis
D. Respiratory alkalosis
Answer: C. Metabolic alkalosis
Bold italics: Loss of gastric H⁺ leads to metabolic alkalosis.
6. A child with recurrent Neisseria infections most likely has
a deficiency in which complement component?
A. C1
B. C3
C. C5–C9
D. Factor H
Answer: C. C5–C9
Bold italics: Terminal complement deficiencies predispose to
Neisseria infections.
7. Which neurotransmitter is decreased in Alzheimer
disease?
A. Dopamine
B. Acetylcholine
, C. Serotonin
D. GABA
Answer: B. Acetylcholine
Bold italics: Loss of cholinergic neurons in the basal nucleus
of Meynert is characteristic.
8. A frameshift mutation is most likely caused by:
A. Deletion of three nucleotides
B. Insertion of one nucleotide
C. Substitution of one nucleotide
D. Deletion of six nucleotides
Answer: B. Insertion of one nucleotide
Bold italics: Insertions or deletions not in multiples of three
cause frameshifts.
9. Which cell type produces surfactant?
A. Type I pneumocytes
B. Type II pneumocytes
C. Alveolar macrophages
D. Clara cells
Answer: B. Type II pneumocytes
Bold italics: Type II pneumocytes synthesize and secrete
surfactant.
Newest With Complete Questions And
Correct Detailed Answers| Brand New
Version!2026 2027
1. A 6-year-old boy presents with recurrent infections and
eczema. Laboratory studies show elevated IgE levels. Which
cytokine is most responsible for IgE class switching?
A. IFN-γ
B. IL-2
C. IL-4
D. IL-12
Answer: C. IL-4
Bold italics: IL-4 promotes B-cell class switching to IgE and
IgG4 and is produced by Th2 cells.
2. A mutation in the dystrophin gene most directly affects
which cellular structure?
A. Sarcomere
B. Z-line
C. Costamere
D. Mitochondria
,Answer: C. Costamere
Bold italics: Dystrophin anchors muscle fibers to the
extracellular matrix via costameres.
3. Which enzyme is inhibited by statins?
A. HMG-CoA synthase
B. HMG-CoA reductase
C. Acetyl-CoA carboxylase
D. LDL receptor
Answer: B. HMG-CoA reductase
Bold italics: Statins inhibit the rate-limiting enzyme in
cholesterol synthesis.
4. A patient with chronic alcoholism develops confusion,
ophthalmoplegia, and ataxia. Which vitamin deficiency is
responsible?
A. Vitamin B6
B. Vitamin B12
C. Thiamine
D. Niacin
Answer: C. Thiamine
Bold italics: Thiamine deficiency causes Wernicke
encephalopathy.
,5. Which acid–base disturbance is expected in a patient with
prolonged vomiting?
A. Metabolic acidosis
B. Respiratory acidosis
C. Metabolic alkalosis
D. Respiratory alkalosis
Answer: C. Metabolic alkalosis
Bold italics: Loss of gastric H⁺ leads to metabolic alkalosis.
6. A child with recurrent Neisseria infections most likely has
a deficiency in which complement component?
A. C1
B. C3
C. C5–C9
D. Factor H
Answer: C. C5–C9
Bold italics: Terminal complement deficiencies predispose to
Neisseria infections.
7. Which neurotransmitter is decreased in Alzheimer
disease?
A. Dopamine
B. Acetylcholine
, C. Serotonin
D. GABA
Answer: B. Acetylcholine
Bold italics: Loss of cholinergic neurons in the basal nucleus
of Meynert is characteristic.
8. A frameshift mutation is most likely caused by:
A. Deletion of three nucleotides
B. Insertion of one nucleotide
C. Substitution of one nucleotide
D. Deletion of six nucleotides
Answer: B. Insertion of one nucleotide
Bold italics: Insertions or deletions not in multiples of three
cause frameshifts.
9. Which cell type produces surfactant?
A. Type I pneumocytes
B. Type II pneumocytes
C. Alveolar macrophages
D. Clara cells
Answer: B. Type II pneumocytes
Bold italics: Type II pneumocytes synthesize and secrete
surfactant.
