A 10-year-old boy develops gross hematuria 2 weeks after strep throat. C3 is low but C4 is normal. Which complement pathway activation mechanism explains this?

• A Alternative pathway activation by planted streptococcal antigens in subepithelial deposits ✓
• B Classical pathway activation via IgG immune complexes in the mesangium
• C Lectin-binding pathway activation by mannan on streptococcal surface
• D Direct C3 convertase mutation causing unregulated complement

Explanation

Post-streptococcal GN features low C3 with normal C4, indicating predominant alternative pathway activation (which consumes C3 but not C4). Streptococcal antigens deposited subepithelially directly activate the alternative pathway. Classical pathway activation would lower both C3 and C4. This pattern of complement consumption is diagnostically important in distinguishing post-infectious GN from SLE nephritis (both C3 and C4 low).

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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