NBME CBSE Actual Test Questions and Answers
2024/2025 – Quiz Bank with Correct Answers (USMLE
Step 1 Preparation)
Question 1:
A 24-year-old man presents with fever, cough, and chest pain. Chest X-ray reveals a right upper
lobe cavitary lesion. A sputum smear shows acid-fast bacilli. Which of the following mechanisms
best explains the primary virulence factor of the causative organism?
A) Inhibition of phagosome-lysosome fusion
B) Production of exotoxin A
C) Formation of IgA protease
D) Capsule-mediated inhibition of complement activation
Correct Answer: A
Rationale: Mycobacterium tuberculosis survives in macrophages by inhibiting
phagosome-lysosome fusion (A), allowing intracellular persistence and granuloma formation.
Exotoxin A (B) is produced by Pseudomonas aeruginosa and inhibits protein synthesis. IgA
protease (C) is produced by Neisseria species to evade mucosal immunity. Capsule-mediated
complement inhibition (D) is characteristic of Streptococcus pneumoniae, preventing
opsonization.
Question 2:
A 45-year-old woman with a history of smoking presents with weight loss and hemoptysis.
Biopsy of a central lung mass shows small cell carcinoma. Which of the following paraneoplastic
syndromes is most commonly associated with this tumor?
A) Cushing syndrome
B) Lambert-Eaton myasthenic syndrome
C) Hypercalcemia
D) Clubbing
Correct Answer: B
Rationale: Small cell lung carcinoma is strongly associated with Lambert-Eaton myasthenic
syndrome (B) due to autoantibodies against voltage-gated calcium channels. Cushing syndrome
(A) is more common with ectopic ACTH production in small cell but less specific than B.
Hypercalcemia (C) is typical of squamous cell carcinoma via PTHrP. Clubbing (D) is nonspecific
and seen in various lung diseases.
Question 3:
,A 6-month-old infant presents with failure to thrive and recurrent infections. Laboratory studies
show elevated very long-chain fatty acids. Which enzyme deficiency is the underlying cause?
A) Peroxisomal very long-chain acyl-CoA synthetase
B) Glucocerebrosidase
C) Hexosaminidase A
D) Phenylalanine hydroxylase
Correct Answer: A
Rationale: Zellweger syndrome, a peroxisomal disorder, results from deficiency in very
long-chain acyl-CoA synthetase (A), leading to accumulation of very long-chain fatty acids and
impaired myelination/infections. Glucocerebrosidase (B) causes Gaucher disease.
Hexosaminidase A (C) causes Tay-Sachs. Phenylalanine hydroxylase (D) causes PKU.
Question 4:
A 32-year-old man is brought to the emergency department after a motor vehicle collision. He
has a compound fracture of the tibia. The wound is irrigated, debrided, and closed. Gram stain
of the wound shows gram-positive cocci in chains. Which organism is most likely responsible for
a subsequent infection?
A) Staphylococcus aureus
B) Streptococcus pyogenes
C) Pseudomonas aeruginosa
D) Clostridium perfringens
Correct Answer: B
Rationale: Group A Streptococcus (Streptococcus pyogenes, B) is a common cause of
necrotizing fasciitis in contaminated wounds, appearing as gram-positive cocci in chains.
Staphylococcus aureus (A) is clusters. Pseudomonas (C) is gram-negative rods. Clostridium (D)
is gram-positive rods and anaerobic.
Question 5:
A 55-year-old man with chronic alcohol use presents with ascites and jaundice. Liver biopsy
shows Mallory bodies and fatty change. Which molecular mechanism is primarily responsible for
the steatosis?
A) Increased NADH/NAD+ ratio inhibiting beta-oxidation
B) Decreased lipoprotein synthesis
C) Enhanced gluconeogenesis
D) Upregulation of SREBP-1c
Correct Answer: A
, Rationale: Alcoholic steatosis results from increased NADH/NAD+ ratio (A) from alcohol
dehydrogenase, inhibiting fatty acid oxidation and promoting triglyceride accumulation.
Decreased lipoprotein (B) is in cirrhosis. Gluconeogenesis (C) is increased but not primary for
steatosis. SREBP-1c (D) is for non-alcoholic.
Question 6:
A newborn is diagnosed with tetralogy of Fallot. Echocardiography shows an overriding aorta
and right ventricular hypertrophy. During embryologic development, failure of which structure to
separate contributes to this defect?
A) Conotruncal septum
B) Endocardial cushions
C) Bulbus cordis
D) Septum primum
Correct Answer: A
Rationale: Tetralogy of Fallot arises from abnormal conotruncal septation (A), leading to
malalignment and the four defects. Endocardial cushions (B) cause AV canal defects. Bulbus
cordis (C) relates to truncus arteriosus. Septum primum (D) to ASD.
Question 7:
A 28-year-old woman presents with fatigue and dyspnea. Laboratory studies show hemoglobin
of 8 g/dL, MCV of 72 fL, and serum ferritin of 10 ng/mL. Which of the following is the most likely
cause of her microcytic anemia?
A) Iron deficiency
B) Thalassemia
C) Anemia of chronic disease
D) Sideroblastic anemia
Correct Answer: A
Rationale: Low ferritin confirms iron deficiency anemia (A), common in women due to menses,
causing microcytic hypochromic RBCs. Thalassemia (B) has normal/high ferritin. Anemia of
chronic disease (C) has normal/low-normal ferritin. Sideroblastic (D) has high ferritin/ringed
sideroblasts.
Question 8:
A 65-year-old man with a history of atrial fibrillation presents with sudden weakness. CT scan
shows a large left MCA infarct. Which neurotransmitter is most implicated in the excitotoxic
neuronal death following ischemia?
A) Glutamate
B) GABA
C) Dopamine
2024/2025 – Quiz Bank with Correct Answers (USMLE
Step 1 Preparation)
Question 1:
A 24-year-old man presents with fever, cough, and chest pain. Chest X-ray reveals a right upper
lobe cavitary lesion. A sputum smear shows acid-fast bacilli. Which of the following mechanisms
best explains the primary virulence factor of the causative organism?
A) Inhibition of phagosome-lysosome fusion
B) Production of exotoxin A
C) Formation of IgA protease
D) Capsule-mediated inhibition of complement activation
Correct Answer: A
Rationale: Mycobacterium tuberculosis survives in macrophages by inhibiting
phagosome-lysosome fusion (A), allowing intracellular persistence and granuloma formation.
Exotoxin A (B) is produced by Pseudomonas aeruginosa and inhibits protein synthesis. IgA
protease (C) is produced by Neisseria species to evade mucosal immunity. Capsule-mediated
complement inhibition (D) is characteristic of Streptococcus pneumoniae, preventing
opsonization.
Question 2:
A 45-year-old woman with a history of smoking presents with weight loss and hemoptysis.
Biopsy of a central lung mass shows small cell carcinoma. Which of the following paraneoplastic
syndromes is most commonly associated with this tumor?
A) Cushing syndrome
B) Lambert-Eaton myasthenic syndrome
C) Hypercalcemia
D) Clubbing
Correct Answer: B
Rationale: Small cell lung carcinoma is strongly associated with Lambert-Eaton myasthenic
syndrome (B) due to autoantibodies against voltage-gated calcium channels. Cushing syndrome
(A) is more common with ectopic ACTH production in small cell but less specific than B.
Hypercalcemia (C) is typical of squamous cell carcinoma via PTHrP. Clubbing (D) is nonspecific
and seen in various lung diseases.
Question 3:
,A 6-month-old infant presents with failure to thrive and recurrent infections. Laboratory studies
show elevated very long-chain fatty acids. Which enzyme deficiency is the underlying cause?
A) Peroxisomal very long-chain acyl-CoA synthetase
B) Glucocerebrosidase
C) Hexosaminidase A
D) Phenylalanine hydroxylase
Correct Answer: A
Rationale: Zellweger syndrome, a peroxisomal disorder, results from deficiency in very
long-chain acyl-CoA synthetase (A), leading to accumulation of very long-chain fatty acids and
impaired myelination/infections. Glucocerebrosidase (B) causes Gaucher disease.
Hexosaminidase A (C) causes Tay-Sachs. Phenylalanine hydroxylase (D) causes PKU.
Question 4:
A 32-year-old man is brought to the emergency department after a motor vehicle collision. He
has a compound fracture of the tibia. The wound is irrigated, debrided, and closed. Gram stain
of the wound shows gram-positive cocci in chains. Which organism is most likely responsible for
a subsequent infection?
A) Staphylococcus aureus
B) Streptococcus pyogenes
C) Pseudomonas aeruginosa
D) Clostridium perfringens
Correct Answer: B
Rationale: Group A Streptococcus (Streptococcus pyogenes, B) is a common cause of
necrotizing fasciitis in contaminated wounds, appearing as gram-positive cocci in chains.
Staphylococcus aureus (A) is clusters. Pseudomonas (C) is gram-negative rods. Clostridium (D)
is gram-positive rods and anaerobic.
Question 5:
A 55-year-old man with chronic alcohol use presents with ascites and jaundice. Liver biopsy
shows Mallory bodies and fatty change. Which molecular mechanism is primarily responsible for
the steatosis?
A) Increased NADH/NAD+ ratio inhibiting beta-oxidation
B) Decreased lipoprotein synthesis
C) Enhanced gluconeogenesis
D) Upregulation of SREBP-1c
Correct Answer: A
, Rationale: Alcoholic steatosis results from increased NADH/NAD+ ratio (A) from alcohol
dehydrogenase, inhibiting fatty acid oxidation and promoting triglyceride accumulation.
Decreased lipoprotein (B) is in cirrhosis. Gluconeogenesis (C) is increased but not primary for
steatosis. SREBP-1c (D) is for non-alcoholic.
Question 6:
A newborn is diagnosed with tetralogy of Fallot. Echocardiography shows an overriding aorta
and right ventricular hypertrophy. During embryologic development, failure of which structure to
separate contributes to this defect?
A) Conotruncal septum
B) Endocardial cushions
C) Bulbus cordis
D) Septum primum
Correct Answer: A
Rationale: Tetralogy of Fallot arises from abnormal conotruncal septation (A), leading to
malalignment and the four defects. Endocardial cushions (B) cause AV canal defects. Bulbus
cordis (C) relates to truncus arteriosus. Septum primum (D) to ASD.
Question 7:
A 28-year-old woman presents with fatigue and dyspnea. Laboratory studies show hemoglobin
of 8 g/dL, MCV of 72 fL, and serum ferritin of 10 ng/mL. Which of the following is the most likely
cause of her microcytic anemia?
A) Iron deficiency
B) Thalassemia
C) Anemia of chronic disease
D) Sideroblastic anemia
Correct Answer: A
Rationale: Low ferritin confirms iron deficiency anemia (A), common in women due to menses,
causing microcytic hypochromic RBCs. Thalassemia (B) has normal/high ferritin. Anemia of
chronic disease (C) has normal/low-normal ferritin. Sideroblastic (D) has high ferritin/ringed
sideroblasts.
Question 8:
A 65-year-old man with a history of atrial fibrillation presents with sudden weakness. CT scan
shows a large left MCA infarct. Which neurotransmitter is most implicated in the excitotoxic
neuronal death following ischemia?
A) Glutamate
B) GABA
C) Dopamine
