A 7-year-old boy develops facial puffiness, dark cola-colored urine, and hypertension two weeks after recovering from a skin infection (impetigo). Complement C3 levels are markedly depressed. Renal biopsy would most likely demonstrate:

Answer and explanation

Correct answer: B. Subepithelial 'humps' on electron microscopy representing IgG and C3 immune complexes

  1. Step 1: Recognize poststreptococcal glomerulonephritis (PSGN): nephritic presentation (cola-colored hematuria, periorbital edema, hypertension) occurring 1 to 4 weeks after group A streptococcal impetigo or pharyngitis with hypocomplementemia (low C3).
  2. Step 2: PSGN is a Type III hypersensitivity disorder caused by circulating immune complexes that deposit along the glomerular basement membrane. Electron microscopy reveals pathognomonic large, dome-shaped subepithelial electron-dense 'humps', with granular IgG and C3 deposition on immunofluorescence ('starry-sky' appearance).
Answer
Subepithelial 'humps' on electron microscopy representing IgG and C3 immune complexes.
Why not the others: Linear IgG deposition along the glomerular basement membrane is diagnostic of anti-GBM disease (Goodpasture syndrome), a Type II hypersensitivity; Thickened capillary loops with 'wire-loop' appearance is typical of diffuse proliferative lupus nephritis (Class IV SLE); Mesangial IgA deposits on immunofluorescence is diagnostic of IgA nephropathy (Berger disease), which typically occurs within days of an upper respiratory infection without latent delay.

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