A 7-year-old boy presents with periorbital edema, dark tea-colored urine, and hypertension (BP 138/88 mmHg). Two weeks earlier, he had an untreated impetigo skin infection. Urinalysis reveals proteinuria (1+), dysmorphic RBCs, and red blood cell casts. What serum serology confirms the diagnosis?

Answer and explanation

Correct answer: D. Depressed serum C3 complement and elevated anti-DNase B titers

  1. Step 1: Acute nephritic syndrome (hematuria, RBC casts, edema, hypertension) following cutaneous streptococcal infection is characteristic of Post-Streptococcal Glomerulonephritis (PSGN).
  2. Step 2: Anti-DNase B titers are the most sensitive serologic marker for prior streptococcal skin infections (ASO is sensitive for pharyngitis).
  3. Step 3: Activation of the alternative complement pathway results in transiently depressed serum C3 levels that normalize within 6-8 weeks.
Answer
Depressed serum C3 complement and elevated anti-DNase B titers.
Why not the others: Normal C3 with elevated IgA is IgA nephropathy; ANA is lupus nephritis; anti-GBM causes Goodpasture syndrome.

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