This is the answer and explanation for a USMLE practice question from the Dr. Ray Makar Facebook page. Try it first, then check your answer below.
The Question
An 8-year-old boy has puffy eyes and cola-colored urine 2 weeks after a sore throat. BP is 138/88 mm Hg. Urinalysis shows RBC casts, serum C3 is low, and ASO titer is elevated. Which best describes the mechanism of his kidney disease?
A. IgE-mediated mast cell degranulation
B. Antibodies against the glomerular basement membrane
C. Direct cytotoxic T-cell attack on glomeruli
D. Deposition of immune complexes with complement activation
E. Direct bacterial invasion of the kidney
Correct Answer: D. Deposition of immune complexes with complement activation
This is post-streptococcal glomerulonephritis, a type III hypersensitivity reaction. Immune complexes made after a group A streptococcal infection deposit in the glomeruli and activate complement, which lowers C3. The result is a nephritic syndrome: hematuria with RBC casts, edema and hypertension. Electron microscopy shows subepithelial humps.
Why the Other Answers Are Wrong
- IgE-mediated: Type I hypersensitivity (allergy, anaphylaxis), not glomerulonephritis.
- Anti-GBM antibodies: Goodpasture syndrome: linear IgG deposits, often with lung hemorrhage; C3 is usually normal.
- Cytotoxic T cells: Type IV hypersensitivity, not the mechanism here.
- Bacterial invasion: The infection is in the throat or skin; the kidney injury is immune-mediated.
High-Yield Pearl
Sore throat 1–3 weeks earlier + cola urine + low C3 = PSGN. Sore throat with hematuria at the same time (1–2 days) = IgA nephropathy.
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