USMLE Step 1 Exam Practice Questions
And Correct Answers (Verified Answers)
Plus Rationale 2026 Q&A| Instant
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1. A 23-year-old medical student presents with fatigue, pallor, and
exertional dyspnea. Laboratory tests show hemoglobin 9.2 g/dL, MCV
68 fL, low serum ferritin, and increased total iron-binding capacity.
Which of the following is the most likely underlying mechanism of his
anemia?
A. Impaired globin chain synthesis
B. Defective DNA synthesis due to folate deficiency
C. Iron deficiency leading to reduced heme synthesis
D. Chronic hemolysis with increased bilirubin production
Answer: C
Iron deficiency reduces heme production, resulting in microcytic
hypochromic anemia characterized by low ferritin and increased TIBC
due to upregulated transferrin synthesis.
2. A 45-year-old woman presents with muscle weakness that worsens
with activity and improves with rest. Antibodies against acetylcholine
receptors are detected. Which structure is primarily affected?
A. Presynaptic voltage-gated calcium channels
B. Postsynaptic nicotinic acetylcholine receptors
C. Muscle sarcoplasmic reticulum calcium pumps
D. Motor neuron cell bodies in the anterior horn
Answer: B
, Myasthenia gravis involves autoimmune destruction of postsynaptic
nicotinic acetylcholine receptors at the neuromuscular junction.
3. A researcher studies a drug that inhibits bacterial cell wall synthesis by
binding penicillin-binding proteins. Which organism is most likely to be
susceptible?
A. Mycoplasma pneumoniae
B. Escherichia coli
C. Chlamydia trachomatis
D. Rickettsia rickettsii
Answer: B
Gram-negative bacteria like Escherichia coli have peptidoglycan cell
walls containing penicillin-binding proteins targeted by beta-lactam
antibiotics.
4. A 60-year-old man with chronic alcoholism presents with confusion,
ataxia, and ophthalmoplegia. MRI shows mammillary body
degeneration. Which vitamin deficiency is responsible?
A. Vitamin B12
B. Vitamin B1
C. Vitamin B6
D. Vitamin B3
Answer: B
Thiamine (B1) deficiency causes Wernicke-Korsakoff syndrome
characterized by neurologic dysfunction and mammillary body
damage.
5. A patient presents with hypercalcemia, kidney stones, and bone pain.
Lab shows elevated PTH. Which bone cell activity is most increased?
A. Osteoblast-mediated bone formation
B. Osteoclast-mediated bone resorption
C. Chondrocyte proliferation in growth plates
D. Fibroblast collagen deposition
Answer: B
PTH increases osteoclast activity indirectly via RANKL stimulation
from osteoblasts, increasing bone resorption and serum calcium.
6. A 5-year-old child presents with a "slapped cheek" rash and mild
anemia. Which virus is most likely responsible?
, A. Parvovirus B19
B. Epstein-Barr virus
C. Rubella virus
D. Varicella-zoster virus
Answer: A
Parvovirus B19 causes erythema infectiosum and can suppress
erythroid precursors leading to transient aplastic crisis.
7. A patient with COPD is started on a drug that inhibits
phosphodiesterase-4. What is the expected effect?
A. Bronchoconstriction
B. Reduced airway inflammation
C. Increased mucus secretion
D. Decreased surfactant production
Answer: B
PDE-4 inhibitors increase cAMP levels, reducing inflammatory
cytokine release in COPD.
8. A 34-year-old woman presents with episodes of tachycardia, sweating,
and headaches. Imaging reveals a tumor in the adrenal medulla.
Which neurotransmitter is primarily secreted?
A. Acetylcholine
B. Norepinephrine
C. Serotonin
D. Dopamine
Answer: B
Pheochromocytomas of the adrenal medulla secrete catecholamines,
primarily norepinephrine.
9. A child with cystic fibrosis has recurrent lung infections with thick
mucus secretions. Which protein is defective?
A. Aquaporin channels
B. CFTR chloride channel
C. Sodium-potassium ATPase
D. Voltage-gated sodium channels
Answer: B
CFTR mutations impair chloride transport leading to dehydrated,
thick mucus secretions.
And Correct Answers (Verified Answers)
Plus Rationale 2026 Q&A| Instant
Download Pdf
1. A 23-year-old medical student presents with fatigue, pallor, and
exertional dyspnea. Laboratory tests show hemoglobin 9.2 g/dL, MCV
68 fL, low serum ferritin, and increased total iron-binding capacity.
Which of the following is the most likely underlying mechanism of his
anemia?
A. Impaired globin chain synthesis
B. Defective DNA synthesis due to folate deficiency
C. Iron deficiency leading to reduced heme synthesis
D. Chronic hemolysis with increased bilirubin production
Answer: C
Iron deficiency reduces heme production, resulting in microcytic
hypochromic anemia characterized by low ferritin and increased TIBC
due to upregulated transferrin synthesis.
2. A 45-year-old woman presents with muscle weakness that worsens
with activity and improves with rest. Antibodies against acetylcholine
receptors are detected. Which structure is primarily affected?
A. Presynaptic voltage-gated calcium channels
B. Postsynaptic nicotinic acetylcholine receptors
C. Muscle sarcoplasmic reticulum calcium pumps
D. Motor neuron cell bodies in the anterior horn
Answer: B
, Myasthenia gravis involves autoimmune destruction of postsynaptic
nicotinic acetylcholine receptors at the neuromuscular junction.
3. A researcher studies a drug that inhibits bacterial cell wall synthesis by
binding penicillin-binding proteins. Which organism is most likely to be
susceptible?
A. Mycoplasma pneumoniae
B. Escherichia coli
C. Chlamydia trachomatis
D. Rickettsia rickettsii
Answer: B
Gram-negative bacteria like Escherichia coli have peptidoglycan cell
walls containing penicillin-binding proteins targeted by beta-lactam
antibiotics.
4. A 60-year-old man with chronic alcoholism presents with confusion,
ataxia, and ophthalmoplegia. MRI shows mammillary body
degeneration. Which vitamin deficiency is responsible?
A. Vitamin B12
B. Vitamin B1
C. Vitamin B6
D. Vitamin B3
Answer: B
Thiamine (B1) deficiency causes Wernicke-Korsakoff syndrome
characterized by neurologic dysfunction and mammillary body
damage.
5. A patient presents with hypercalcemia, kidney stones, and bone pain.
Lab shows elevated PTH. Which bone cell activity is most increased?
A. Osteoblast-mediated bone formation
B. Osteoclast-mediated bone resorption
C. Chondrocyte proliferation in growth plates
D. Fibroblast collagen deposition
Answer: B
PTH increases osteoclast activity indirectly via RANKL stimulation
from osteoblasts, increasing bone resorption and serum calcium.
6. A 5-year-old child presents with a "slapped cheek" rash and mild
anemia. Which virus is most likely responsible?
, A. Parvovirus B19
B. Epstein-Barr virus
C. Rubella virus
D. Varicella-zoster virus
Answer: A
Parvovirus B19 causes erythema infectiosum and can suppress
erythroid precursors leading to transient aplastic crisis.
7. A patient with COPD is started on a drug that inhibits
phosphodiesterase-4. What is the expected effect?
A. Bronchoconstriction
B. Reduced airway inflammation
C. Increased mucus secretion
D. Decreased surfactant production
Answer: B
PDE-4 inhibitors increase cAMP levels, reducing inflammatory
cytokine release in COPD.
8. A 34-year-old woman presents with episodes of tachycardia, sweating,
and headaches. Imaging reveals a tumor in the adrenal medulla.
Which neurotransmitter is primarily secreted?
A. Acetylcholine
B. Norepinephrine
C. Serotonin
D. Dopamine
Answer: B
Pheochromocytomas of the adrenal medulla secrete catecholamines,
primarily norepinephrine.
9. A child with cystic fibrosis has recurrent lung infections with thick
mucus secretions. Which protein is defective?
A. Aquaporin channels
B. CFTR chloride channel
C. Sodium-potassium ATPase
D. Voltage-gated sodium channels
Answer: B
CFTR mutations impair chloride transport leading to dehydrated,
thick mucus secretions.
