Two weeks after pharyngitis, an 8-year-old boy develops dark urine, periorbital edema, and hypertension. Serum C3 is markedly decreased. Which additional complement finding supports a diagnosis of post-streptococcal glomerulonephritis rather than lupus nephritis?

• A Low C3 with low C4
• B Low C3 with normal C4 ✓
• C Low C4 with normal C3
• D Normal C3 with normal C4

Explanation

Post-streptococcal glomerulonephritis activates the alternative complement pathway, so C3 falls while C4, which is consumed only by the classical and lectin pathways, remains normal. In active lupus nephritis, classical pathway activation consumes both C3 and C4, so a normal C4 argues strongly against lupus and favors postinfectious GN in this hypocomplementemic child.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.