USMLE Step 1 Block (5 High Yield Questions, Answers with Explainations and Timer mode.

USMLE Step 1 Practice Block

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1. A 2-year-old boy has developmental delay, a musty body odor, and hypopigmentation. Which reaction is impaired?

Tyrosine → DOPA Phenylalanine → Tyrosine Homogentisic acid breakdown Tryptophan → Serotonin

Correct Answer: B — Phenylalanine → Tyrosine

Diagnosis: Phenylketonuria (PKU)

Mechanism:
Deficiency of phenylalanine hydroxylase → ↑ phenylalanine → neurotoxicity

Key Features:
• Musty odor
• Developmental delay
• Hypopigmentation

Why others are wrong:
A → Albinism
C → Alkaptonuria
D → Not related

2. A patient with HIV (CD4 120) presents with dry cough and interstitial infiltrates. Silver stain shows cysts. Organism?

Candida Pneumocystis jirovecii Histoplasma Aspergillus

Correct Answer: B — Pneumocystis jirovecii

Key clues:
• CD4 < 200
• Dry cough
• Silver stain cysts

Diagnosis: Opportunistic pneumonia

3. A patient develops cough and angioedema after starting antihypertensive therapy. Mechanism?

↑ Bradykinin ↓ Aldosterone ↑ Renin Na retention

Correct Answer: A — Increased Bradykinin

ACE inhibitors block bradykinin breakdown → accumulation → cough + angioedema.

4. Chronic NSAID use leads to decreased GFR. Why?

Afferent dilation Afferent constriction Efferent dilation Increased filtration

Correct Answer: B — Afferent constriction

NSAIDs ↓ prostaglandins → afferent constriction → ↓ GFR.

5. A patient has hyperthyroidism with exophthalmos. Mechanism?

Thyroid destruction TSH receptor antibodies Iodine deficiency Low T3

Correct Answer: B — TSH receptor antibodies

Graves disease → antibodies stimulate TSH receptor → ↑ T3/T4 → hyperthyroidism.