A 28-year-old man from rural Maharashtra presents with sudden onset gross hematuria, facial puffiness, and hypertension (160/100 mmHg) 10 days after a throat infection. Urine shows RBC casts and dysmorphic RBCs. Serum creatinine is 1.2 mg/dL. Complement C3 levels are low. Kidney biopsy shows subepithelial 'hump' deposits on electron microscopy. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Post-Infectious Glomerulonephritis (PIGN) ### Key Clinical Features **Key Point:** Post-infectious GN (most commonly post-streptococcal) is the classic acute nephritic syndrome in children and young adults, typically 1–3 weeks after group A streptococcal pharyngitis or skin infection. ### Diagnostic Hallmarks | Feature | PIGN | IgA | MPGN-I | Lupus | |---------|------|-----|--------|-------| | **Timing after infection** | 1–3 weeks | Variable, no clear lag | No clear lag | No clear lag | | **C3 level** | ↓↓ (classic) | Normal | ↓ (variable) | ↓ (with C4↓) | | **EM finding** | Subepithelial 'humps' | IgA deposits | Dense intramembranous | Electron-dense deposits | | **Throat infection** | Common antecedent | No clear link | No clear link | No clear link | | **Age** | Children/young adults | Young adults | Variable | Young women | ### Pathology Explanation 1. **Subepithelial 'hump' deposits** are pathognomonic for PIGN — these are immune complexes (streptococcal Ag + IgG/IgM) deposited on the outer (epithelial) side of the GBM. 2. **Low C3** reflects consumption by immune complex activation via the classical complement pathway. 3. **Streptococcal antigen cross-reactivity** with glomerular antigens (e.g., streptococcal hyaluronate mimics glomerular hyaluronate) drives the response. **High-Yield:** The **combination of acute nephritic syndrome + recent pharyngitis + low C3 + subepithelial humps = PIGN** until proven otherwise. ### Clinical Pearl **Clinical Pearl:** Most cases of PIGN resolve spontaneously within 4–6 weeks with supportive care (salt restriction, diuretics, antihypertensives). Prognosis in children is excellent; adults may have residual proteinuria or CKD. ### Prognosis - **Children:** >95% recover completely. - **Adults:** ~50% recover completely; ~30% have persistent proteinuria or mild renal impairment. - **Rapidly progressive GN** (RPGN) is rare but indicates poor prognosis if present.