USMLE Step 1 Self-Assessment Exam
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Answers) Plus Rationales 2026 Q&A | Instant
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Question 1
A 24-year-old man presents with fatigue and pallor. Lab results show
microcytic hypochromic anemia. Which defect is most likely
responsible?
A. Vitamin B12 deficiency
B. Iron deficiency
C. Folate deficiency
D. Aplastic anemia
Correct answer: B. Iron deficiency
Rationale: Iron deficiency impairs hemoglobin synthesis, leading to
microcytic, hypochromic anemia. Vitamin B12 and folate deficiencies
cause macrocytic anemia, while aplastic anemia presents with
pancytopenia.
,Question 2
Which enzyme is deficient in patients with classic galactosemia?
A. Galactokinase
B. Galactose-1-phosphate uridyltransferase
C. Lactase
D. Aldolase B
Correct answer: B. Galactose-1-phosphate uridyltransferase
Rationale: Classic galactosemia is due to deficiency of GALT, leading to
accumulation of galactose-1-phosphate and severe neonatal
symptoms.
Question 3
A patient with emphysema is found to have a genetic deficiency leading
to increased neutrophil elastase activity. Which protein is deficient?
A. Alpha-1 antitrypsin
B. Cystic fibrosis transmembrane regulator
C. Surfactant protein A
D. Collagen type IV
,Correct answer: A. Alpha-1 antitrypsin
Rationale: Alpha-1 antitrypsin inhibits neutrophil elastase in the lungs.
Deficiency leads to panacinar emphysema.
Question 4
Which neurotransmitter is primarily involved in Parkinson disease
pathology?
A. Dopamine
B. Serotonin
C. GABA
D. Acetylcholine
Correct answer: A. Dopamine
Rationale: Parkinson disease is caused by degeneration of
dopaminergic neurons in the substantia nigra.
Question 5
Which vitamin deficiency is associated with Wernicke encephalopathy?
A. Vitamin B1 (thiamine)
B. Vitamin B6
, C. Vitamin B12
D. Vitamin C
Correct answer: A. Vitamin B1 (thiamine)
Rationale: Thiamine deficiency leads to Wernicke encephalopathy
characterized by confusion, ophthalmoplegia, and ataxia.
Question 6
Which structure is responsible for producing cerebrospinal fluid?
A. Choroid plexus
B. Dura mater
C. Arachnoid granulations
D. Pia mater
Correct answer: A. Choroid plexus
Rationale: The choroid plexus produces CSF within the ventricles.
Question 7
Which immunoglobulin is most abundant in serum?
A. IgA
B. IgG
Questions With Correct Answers (Verified
Answers) Plus Rationales 2026 Q&A | Instant
Download Pdf
Question 1
A 24-year-old man presents with fatigue and pallor. Lab results show
microcytic hypochromic anemia. Which defect is most likely
responsible?
A. Vitamin B12 deficiency
B. Iron deficiency
C. Folate deficiency
D. Aplastic anemia
Correct answer: B. Iron deficiency
Rationale: Iron deficiency impairs hemoglobin synthesis, leading to
microcytic, hypochromic anemia. Vitamin B12 and folate deficiencies
cause macrocytic anemia, while aplastic anemia presents with
pancytopenia.
,Question 2
Which enzyme is deficient in patients with classic galactosemia?
A. Galactokinase
B. Galactose-1-phosphate uridyltransferase
C. Lactase
D. Aldolase B
Correct answer: B. Galactose-1-phosphate uridyltransferase
Rationale: Classic galactosemia is due to deficiency of GALT, leading to
accumulation of galactose-1-phosphate and severe neonatal
symptoms.
Question 3
A patient with emphysema is found to have a genetic deficiency leading
to increased neutrophil elastase activity. Which protein is deficient?
A. Alpha-1 antitrypsin
B. Cystic fibrosis transmembrane regulator
C. Surfactant protein A
D. Collagen type IV
,Correct answer: A. Alpha-1 antitrypsin
Rationale: Alpha-1 antitrypsin inhibits neutrophil elastase in the lungs.
Deficiency leads to panacinar emphysema.
Question 4
Which neurotransmitter is primarily involved in Parkinson disease
pathology?
A. Dopamine
B. Serotonin
C. GABA
D. Acetylcholine
Correct answer: A. Dopamine
Rationale: Parkinson disease is caused by degeneration of
dopaminergic neurons in the substantia nigra.
Question 5
Which vitamin deficiency is associated with Wernicke encephalopathy?
A. Vitamin B1 (thiamine)
B. Vitamin B6
, C. Vitamin B12
D. Vitamin C
Correct answer: A. Vitamin B1 (thiamine)
Rationale: Thiamine deficiency leads to Wernicke encephalopathy
characterized by confusion, ophthalmoplegia, and ataxia.
Question 6
Which structure is responsible for producing cerebrospinal fluid?
A. Choroid plexus
B. Dura mater
C. Arachnoid granulations
D. Pia mater
Correct answer: A. Choroid plexus
Rationale: The choroid plexus produces CSF within the ventricles.
Question 7
Which immunoglobulin is most abundant in serum?
A. IgA
B. IgG
