A 6-year-old boy from Delhi presents with a 2-week history of impetigo on his left knee following a minor abrasion. The lesions are non-bullous, with honey-crusted appearance and surrounding erythema. Gram stain of the pus shows gram-positive cocci in chains. Two weeks after the onset of skin lesions, he develops periorbital edema, tea-colored urine, and mild hypertension (BP 110/70 mmHg). Serum creatinine is mildly elevated at 1.2 mg/dL. What is the most likely renal complication?

Explanation

## Post-Streptococcal Glomerulonephritis (PSGN) After GAS Impetigo **Key Point:** Post-streptococcal glomerulonephritis (PSGN) is the most common acute glomerulonephritis in children worldwide, typically occurring 1–3 weeks after GAS skin infection (impetigo) or 1–2 weeks after pharyngitis. It presents with hematuria, proteinuria, hypertension, and edema. ### Clinical Features Pointing to PSGN | Feature | This Patient | PSGN Typical | Significance | |---------|-------------|-------------|---------------| | **Age** | 6 years | Peak 3–12 years | Classic pediatric presentation | | **Preceding infection** | GAS impetigo (skin) | GAS pharyngitis or skin | Latency 1–3 weeks post-skin, 1–2 weeks post-throat | | **Presentation** | Tea-colored urine, periorbital edema | Hematuria, edema, hypertension | Hallmark of PSGN | | **Hypertension** | Mild (110/70) | Present in ~50% | Due to volume expansion and renin activation | | **Renal function** | Mildly elevated Cr | Mild elevation common | Usually reversible; <5% progress to ESRD | | **Pathology** | Acute proliferative GN | Diffuse endocapillary proliferation | Immune complex (IgG, C3) deposition | **High-Yield:** The latency of 2 weeks between GAS impetigo and renal symptoms is pathognomonic for PSGN. Throat infections have a shorter latency (1–2 weeks); skin infections have a longer latency (1–3 weeks). ### Pathogenesis of PSGN ```mermaid flowchart TD A[GAS skin infection]:::outcome --> B[Nephritogenic strains M1, M12, M49, M60, M61]:::outcome B --> C[Immune complex formation]:::action C --> D[IgG + IgM + C3 deposition in glomeruli]:::action D --> E[Endocapillary proliferation]:::outcome E --> F[Hematuria, proteinuria, hypertension]:::outcome F --> G{Prognosis?}:::decision G -->|Children| H[>95% complete recovery]:::outcome G -->|Adults| I[Slower recovery, 5-10% ESRD]:::outcome ``` **Mnemonic — PSGN Features (HEME):** - **H**ematuria (tea-colored urine, RBC casts) - **E**dema (periorbital, peripheral) - **M**ild hypertension - **E**levated creatinine (usually mild, transient) ### Immunological Basis **Key Point:** PSGN is an **immune complex-mediated glomerulonephritis** (Type III hypersensitivity). Circulating immune complexes (antigen-antibody) deposit in the glomerular basement membrane and mesangium, activating complement (especially C3) and triggering inflammation. - **Antigen:** GAS cell wall components (hyaluronic acid, M protein epitopes) - **Antibody:** IgG and IgM against streptococcal antigens - **Complement:** C3 and C4 consumption (C3 is depressed in acute phase; recovers in 8–12 weeks) - **Histology:** Diffuse endocapillary proliferation with "humps" (subepithelial immune deposits on electron microscopy) ### Distinguishing PSGN from Other GN Types | Feature | PSGN (Acute Proliferative) | IgA Nephropathy | RPGN | MPGN Type II | |---------|---------------------------|-----------------|------|---------------| | **Latency after GAS** | 1–3 weeks | Variable; not GAS-linked | Days to weeks | Not GAS-associated | | **Age of onset** | Children > adults | Young adults | Any age | Children/young adults | | **Hematuria** | Gross (tea-colored) | Gross or microscopic | Gross ± RBC casts | Microscopic | | **Hypertension** | ~50% | Rare | Common | Rare | | **Renal function** | Mild elevation | Slowly progressive | Rapid decline (days–weeks) | Progressive | | **Pathology** | Endocapillary proliferation, C3 dominant | IgA-dominant deposits | Crescent formation, ANCA | Dense deposits ("ribbon-like") | | **Prognosis** | >95% recovery in children | Slow progression | Poor without treatment | Variable | | **C3 level** | Depressed acutely | Normal | Normal | Low | **Clinical Pearl:** The **depressed C3 level** (normal C4) in acute PSGN is a key diagnostic clue and helps distinguish it from IgA nephropathy (normal C3) and MPGN Type II (persistently low C3). C3 recovers within 8–12 weeks in PSGN. ### Why This Is PSGN, Not Other Diagnoses 1. **Timing:** 2 weeks post-GAS impetigo is classic for PSGN (latency 1–3 weeks for skin infection). 2. **Age:** 6 years is peak incidence for PSGN in children. 3. **Presentation:** Tea-colored urine (gross hematuria) + periorbital edema + mild hypertension = PSGN triad. 4. **Gram stain:** Gram-positive cocci in chains = Streptococcus (confirmed GAS etiology). 5. **Renal function:** Mild, reversible elevation of creatinine (not rapid decline as in RPGN). **Warning:** Do not confuse PSGN with IgA nephropathy — IgA is NOT associated with GAS infection and has a different age of onset (young adults) and slower, progressive course.