A 10-year-old boy presents 10 days after an episode of impetigo with cola-colored urine, facial edema, and hypertension. Urine microscopy shows RBC casts and dysmorphic RBCs. Complement C3 is low, C4 is normal. ASOT is elevated. Which finding differentiates Post-Streptococcal Glomerulonephritis (PSGN) from Lupus Nephritis?
- A Low C3 with normal C4 (alternative pathway activation) in PSGN vs. low C3 and low C4 in lupus ✓
- B Elevated ASOT in PSGN vs. negative ASOT in lupus
- C RBC casts present only in PSGN
- D Cola-colored urine occurs only in PSGN
Explanation
The complement pattern is diagnostically important: PSGN activates the alternative complement pathway (initially) — C3 is low but C4 is typically normal in the acute phase. In Systemic Lupus Erythematosus (lupus nephritis), the classical pathway is predominantly activated by immune complexes — BOTH C3 and C4 are low. This complement consumption pattern (low C3+low C4 vs. low C3+normal C4) is a key differentiating point. Elevated ASOT supports streptococcal infection but is not specific; ASOT may also be elevated in other streptococcal-associated nephritides. RBC casts occur in both PSGN and lupus nephritis. Cola/tea-colored urine is hematuria and is nonspecific to PSGN.
Reference: Ghai Essential Pediatrics, 10th ed.
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Written and medically reviewed by the StethoPrep medical team.