A 10-year-old boy presents with macroscopic hematuria following an upper respiratory tract infection (synpharyngitic hematuria — hematuria within 1-3 days of throat infection). Renal biopsy shows mesangial IgA deposits. Which complement pathway is primarily activated in IgA nephropathy?

• A Classical complement pathway via C1q
• B Lectin complement pathway via MBL-MASP ✓
• C Alternative complement pathway via properdin
• D Terminal complement pathway via C5b-9 membrane attack complex

Explanation

In IgA nephropathy, galactose-deficient IgA1 (Gd-IgA1) deposited in the mesangium activates the lectin pathway of complement via mannose-binding lectin (MBL) and/or galectin recognizing the aberrantly glycosylated IgA. C1q is typically absent on immunofluorescence (excluding classical pathway), while complement C3 is present. This distinguishes IgA nephropathy from lupus nephritis (which uses classical pathway, C1q present). The lectin pathway activation contributes to mesangial injury. Synpharyngitic hematuria (within 1-3 days) differentiates it from post-streptococcal GN (2-4 weeks after infection).

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.