A 12-year-old boy develops hematuria and proteinuria 1 week after streptococcal skin infection. Renal biopsy electron microscopy shows subepithelial 'hump-shaped' electron-dense deposits. Which complement pathway is primarily activated?
- A Classical pathway — C1q deposits visible on immunofluorescence
- B Alternative pathway — C3 deposition without C1q or C4, mediated by streptococcal proteins activating properdin ✓
- C Lectin pathway — MBL-MASP complex activated by streptococcal carbohydrates
- D Terminal complement pathway only — MAC deposition on podocytes
Correct answer: B. Alternative pathway — C3 deposition without C1q or C4, mediated by streptococcal proteins activating properdin
Explanation
Post-streptococcal glomerulonephritis activates primarily the alternative complement pathway. Immunofluorescence shows coarse granular C3 and IgG deposits (the 'starry sky' pattern), but notably C1q is typically absent or minimal, indicating the alternative rather than classical pathway. Certain streptococcal proteins (SPEB/SpeB, GAPDH) directly activate the alternative pathway via properdin, driving C3 deposition and the characteristic subepithelial humps seen on EM.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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