A 7-year-old presents with hematuria, oliguria, and edema 2 weeks after a throat infection. Serum complement C3 is low while C4 is normal. The pattern of complement consumption (low C3, normal C4) suggests activation via:

• A Classical pathway
• B Alternative pathway ✓
• C Lectin pathway
• D Terminal (membrane attack complex) pathway only

Explanation

Post-streptococcal glomerulonephritis activates complement predominantly via the alternative pathway (and possibly lectin pathway), consuming C3 while sparing C4. The classical pathway (activated by antigen-antibody complexes) consumes both C4 and C3. In SLE nephritis, classical pathway activation reduces both C3 and C4. The selective reduction of C3 with normal C4 points to 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.