A 10-year-old boy with recent throat infection develops hematuria, edema, and hypertension. Complement C3 is low but C4 is normal. Renal biopsy shows 'humps' on electron microscopy. Which complement pathway is primarily activated in this condition?

• A Classical pathway, activated by IgG-containing immune complexes
• B Lectin pathway, activated by mannose residues on streptococcal surface
• C Terminal pathway, with isolated C5-C9 activation without upstream consumption
• D Alternative pathway, activated by bacterial cell wall components ✓

Explanation

Post-streptococcal glomerulonephritis activates primarily the alternative complement pathway, explaining the selective depression of C3 with normal C4 levels (C4 is specific to the classical and lectin pathways). The deposited immune complexes (with streptococcal nephritogenic antigens such as nephritis-associated plasmin receptor and SpeB) also directly activate the alternative pathway. The subepithelial 'humps' on electron microscopy represent large immune complex deposits characteristic of this condition. Low C3 with normal C4 is the classic serological footprint distinguishing alternative pathway activation.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.