Two weeks after pharyngitis, a 9-year-old boy develops nephritic syndrome. Serum C3 is low. Serial measurement of complement is used to distinguish post-streptococcal glomerulonephritis from membranoproliferative glomerulonephritis. In post-streptococcal glomerulonephritis, the expected behavior of serum C3 is:

• A Persistently low throughout follow-up
• B Low C3 with persistently very low C4
• C Normalizes within 6 to 8 weeks ✓
• D Always normal from onset

Explanation

Hypocomplementemia in post-streptococcal glomerulonephritis is transient, with B3 returning to normal within 6 to 8 weeks; persistence beyond this period suggests an alternative diagnosis such as MPGN, B3 glomerulopathy, or lupus nephritis. Option A describes MPGN and dense deposit disease, where B3 remains depressed. Option B fits classical pathway activation with low B4, typical of lupus and cryoglobulinemia rather than post-streptococcal disease, in which B4 is usually normal because the alternative pathway predominates.

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.