USMLE Step 1 Exam Practice Questions
And Correct Answers (Verified Answers)
Plus Rationales 2026 Q&A | Instant
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1. A 45-year-old man has chronic alcohol use and presents with
confusion and ataxia. Deficiency of which vitamin is most likely
responsible?
A) Vitamin A
B) Vitamin B1 (Thiamine)
C) Vitamin B6 (Pyridoxine)
D) Vitamin B12
Answer: B) Vitamin B1 (Thiamine)
Rationale: Thiamine deficiency causes Wernicke encephalopathy
characterized by confusion, ataxia, and ophthalmoplegia, commonly
seen in chronic alcoholism.
2. Which organ is the primary site of gluconeogenesis?
A) Brain
B) Liver
C) Skeletal muscle
D) Adipose tissue
Answer: B) Liver
Rationale: The liver is the major site of gluconeogenesis, especially
during fasting states.
3. A mutation in which gene is most associated with cystic fibrosis?
A) CFTR
B) BRCA1
C) APC
D) p53
, Answer: A) CFTR
Rationale: CFTR gene mutation leads to defective chloride transport
causing thick mucus secretions.
4. Which hypersensitivity reaction is mediated by IgE?
A) Type I
B) Type II
C) Type III
D) Type IV
Answer: A) Type I
Rationale: Type I hypersensitivity involves IgE-mediated mast cell
degranulation (e.g., anaphylaxis).
5. The primary neurotransmitter released at the neuromuscular junction
is:
A) Dopamine
B) Acetylcholine
C) GABA
D) Serotonin
Answer: B) Acetylcholine
Rationale: Acetylcholine is released at the neuromuscular junction to
activate nicotinic receptors.
6. Which structure gives rise to the aortic arch?
A) First pharyngeal arch
B) Third pharyngeal arch
C) Fourth pharyngeal arch
D) Sixth pharyngeal arch
Answer: C) Fourth pharyngeal arch
Rationale: The left fourth pharyngeal arch contributes to the aortic
arch.
7. Which enzyme is deficient in Tay-Sachs disease?
A) Hexosaminidase A
B) Glucocerebrosidase
C) Sphingomyelinase
D) Arylsulfatase A
Answer: A) Hexosaminidase A
, Rationale: Deficiency leads to GM2 ganglioside accumulation causing
neurodegeneration.
8. Which electrolyte is primarily responsible for resting membrane
potential?
A) Calcium
B) Sodium
C) Potassium
D) Chloride
Answer: C) Potassium
Rationale: Potassium leak channels maintain resting membrane
potential.
9. Which immunoglobulin crosses the placenta?
A) IgA
B) IgE
C) IgG
D) IgM
Answer: C) IgG
Rationale: IgG is actively transported across the placenta for passive
immunity.
10. The site of action of loop diuretics is:
A) Proximal tubule
B) Thick ascending loop of Henle
C) Distal convoluted tubule
D) Collecting duct
Answer: B) Thick ascending loop of Henle
Rationale: Loop diuretics inhibit Na-K-2Cl transporter in the thick
ascending limb.
11. Which tumor is associated with EBV infection?
A) Burkitt lymphoma
B) Osteosarcoma
C) Melanoma
D) Glioblastoma
Answer: A) Burkitt lymphoma
Rationale: EBV is linked to Burkitt lymphoma via c-MYC translocation.
And Correct Answers (Verified Answers)
Plus Rationales 2026 Q&A | Instant
Download Pdf
1. A 45-year-old man has chronic alcohol use and presents with
confusion and ataxia. Deficiency of which vitamin is most likely
responsible?
A) Vitamin A
B) Vitamin B1 (Thiamine)
C) Vitamin B6 (Pyridoxine)
D) Vitamin B12
Answer: B) Vitamin B1 (Thiamine)
Rationale: Thiamine deficiency causes Wernicke encephalopathy
characterized by confusion, ataxia, and ophthalmoplegia, commonly
seen in chronic alcoholism.
2. Which organ is the primary site of gluconeogenesis?
A) Brain
B) Liver
C) Skeletal muscle
D) Adipose tissue
Answer: B) Liver
Rationale: The liver is the major site of gluconeogenesis, especially
during fasting states.
3. A mutation in which gene is most associated with cystic fibrosis?
A) CFTR
B) BRCA1
C) APC
D) p53
, Answer: A) CFTR
Rationale: CFTR gene mutation leads to defective chloride transport
causing thick mucus secretions.
4. Which hypersensitivity reaction is mediated by IgE?
A) Type I
B) Type II
C) Type III
D) Type IV
Answer: A) Type I
Rationale: Type I hypersensitivity involves IgE-mediated mast cell
degranulation (e.g., anaphylaxis).
5. The primary neurotransmitter released at the neuromuscular junction
is:
A) Dopamine
B) Acetylcholine
C) GABA
D) Serotonin
Answer: B) Acetylcholine
Rationale: Acetylcholine is released at the neuromuscular junction to
activate nicotinic receptors.
6. Which structure gives rise to the aortic arch?
A) First pharyngeal arch
B) Third pharyngeal arch
C) Fourth pharyngeal arch
D) Sixth pharyngeal arch
Answer: C) Fourth pharyngeal arch
Rationale: The left fourth pharyngeal arch contributes to the aortic
arch.
7. Which enzyme is deficient in Tay-Sachs disease?
A) Hexosaminidase A
B) Glucocerebrosidase
C) Sphingomyelinase
D) Arylsulfatase A
Answer: A) Hexosaminidase A
, Rationale: Deficiency leads to GM2 ganglioside accumulation causing
neurodegeneration.
8. Which electrolyte is primarily responsible for resting membrane
potential?
A) Calcium
B) Sodium
C) Potassium
D) Chloride
Answer: C) Potassium
Rationale: Potassium leak channels maintain resting membrane
potential.
9. Which immunoglobulin crosses the placenta?
A) IgA
B) IgE
C) IgG
D) IgM
Answer: C) IgG
Rationale: IgG is actively transported across the placenta for passive
immunity.
10. The site of action of loop diuretics is:
A) Proximal tubule
B) Thick ascending loop of Henle
C) Distal convoluted tubule
D) Collecting duct
Answer: B) Thick ascending loop of Henle
Rationale: Loop diuretics inhibit Na-K-2Cl transporter in the thick
ascending limb.
11. Which tumor is associated with EBV infection?
A) Burkitt lymphoma
B) Osteosarcoma
C) Melanoma
D) Glioblastoma
Answer: A) Burkitt lymphoma
Rationale: EBV is linked to Burkitt lymphoma via c-MYC translocation.
