A 38-year-old man presents with bilateral flank pain, haematuria, and a serum creatinine of 3.2 mg/dL over 3 weeks. He had an upper respiratory infection 3 weeks ago. Urinalysis shows 3+ blood, 2+ protein, and RBC casts. Complement C3 is 62 mg/dL (low), C4 is 18 mg/dL (normal). ANCA, anti-GBM, ANA, anti-dsDNA are all negative. Anti-streptolysin O (ASO) titre is 180 Todd units (borderline). What is the most likely diagnosis and the expected course?
- A IgA nephropathy — episodic haematuria concurrent with infections, complement typically normal, chronic course
- B Membranoproliferative GN (MPGN) Type I — low C3, can be persistent, associated with hepatitis C
- C Lupus nephritis Class III — selective C3 depression with positive ANA expected
- D Post-streptococcal glomerulonephritis — low C3 with normal C4, typically self-limiting with full recovery in children/adults ✓
Explanation
Post-streptococcal GN (PSGN) characteristically presents 1–3 weeks after pharyngitis or 3–6 weeks after skin infection with haematuria, proteinuria, RBC casts, and hypocomplementaemia — selectively depressed C3 with normal C4 (activation of the alternative complement pathway). C3 typically normalises within 8 weeks; persistent low C3 beyond 8 weeks should prompt investigation for MPGN or C3 glomerulopathy. The ASO titre may be borderline with throat PSGN; anti-DNase B is more sensitive. PSGN in adults has a slightly less favourable prognosis than in children (10–20% develop progressive renal impairment) but most recover. IgA nephropathy causes gross haematuria during infections with typically normal complement. Lupus nephritis would have positive ANA/dsDNA.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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