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 honey-crusted and non-bullous. Culture from the purulent exudate grows β-hemolytic cocci that are Gram-positive, catalase-negative, and PYR-positive. Two weeks later, his mother reports cola-colored urine and mild ankle swelling. Urinalysis shows 2+ proteinuria, RBC casts, and dysmorphic RBCs. What is the most likely diagnosis of the renal complication?

Explanation

## Post-Streptococcal Glomerulonephritis (PSGN): Clinical Presentation & Pathogenesis ### Organism Identification **Key Point:** The isolate is Group A Streptococcus (GAS): - **Gram-positive cocci** (chains) - **β-hemolytic** on blood agar - **Catalase-negative** (distinguishes from Staphylococcus aureus) - **PYR-positive** (pyrrolidonyl arylamidase test; GAS and Enterococcus are PYR+) ### PSGN: Epidemiology & Timing | Feature | Pyoderma-Associated PSGN | Pharyngitis-Associated PSGN | |---------|--------------------------|-----------------------------| | **Latency** | 3–6 weeks (range: 10 days–3 months) | 1–3 weeks (range: 1–4 weeks) | | **Common in** | Tropical/subtropical regions (India, Africa) | Temperate climates | | **Presentation** | Impetigo → hematuria | Pharyngitis → hematuria | | **GAS M-types** | M49, M57, M60 (nephritogenic) | M1, M12, M4 (nephritogenic) | | **Prognosis** | Better (lower mortality) | Slightly worse in children | **Clinical Pearl:** This patient's timeline (impetigo 2 weeks ago → hematuria now) is classic for pyoderma-associated PSGN, which has a latency of 3–6 weeks but can present as early as 10 days post-infection. ### Pathophysiology of PSGN ```mermaid flowchart TD A[GAS skin/throat infection]:::outcome --> B[Immune complex formation]:::outcome B --> C[Deposition in glomerular basement membrane]:::outcome C --> D[Complement activation<br/>C3 consumption]:::outcome D --> E[Glomerular inflammation]:::outcome E --> F[Hematuria, proteinuria,<br/>RBC casts]:::outcome F --> G{Clinical course?}:::decision G -->|Children| H[Complete recovery<br/>95% cases]:::action G -->|Adults| I[Chronic kidney disease<br/>risk 5-10%]:::action ``` ### Laboratory & Clinical Findings in PSGN **Key Point:** PSGN is a post-infectious, immune-complex-mediated glomerulonephritis characterized by: 1. **Serological markers of recent GAS infection:** - Elevated anti-streptolysin O (ASO) titer - Elevated anti-DNase B titer - Positive throat/skin culture (if still positive) 2. **Urinalysis (as in this case):** - Hematuria (dysmorphic RBCs, RBC casts) ← **hallmark** - Proteinuria (usually <2 g/day) - Pyuria (may be present) 3. **Serum complement:** - **Low C3** (most sensitive finding) - Normal C4 - Transient depression; normalizes by 8–12 weeks 4. **Renal function:** - Mild elevation in creatinine (usually reversible) - Oliguria in severe cases **High-Yield:** Cola-colored urine + dysmorphic RBCs + RBC casts = glomerulonephritis. When preceded by GAS infection (skin or throat), PSGN is the diagnosis until proven otherwise. ### Differential Diagnosis: Why Not the Other Options? | Diagnosis | Key Distinguishing Feature | |-----------|---------------------------| | **IgA nephropathy** | No temporal link to GAS; hematuria often gross but RBC casts less common; C3 normal; M-protein on immunofluorescence | | **Acute interstitial nephritis (AIN)** | Caused by drug hypersensitivity (NSAIDs, antibiotics), not infection; WBC casts > RBC casts; eosinophiluria common; normal complement | | **Lupus nephritis** | Requires systemic lupus erythematosus (SLE) diagnosis; positive ANA, anti-dsDNA; low C3 + low C4 (not just C3); no recent GAS infection trigger | **Mnemonic:** **PSGN = POST-STREP** — Post-infectious, Streptococcal, Glomerulonephritis; presents 1–6 weeks after GAS infection (throat or skin) with hematuria, proteinuria, low C3, and excellent prognosis in children. ### Management & Prognosis - **Supportive care:** Fluid restriction, salt restriction, antihypertensives if needed - **Antibiotics:** Complete the GAS course if not already done - **Immunosuppression:** NOT indicated in uncomplicated PSGN - **Prognosis:** >95% of children recover completely; proteinuria and hematuria resolve within weeks to months [cite:Harrison 21e Ch 329; Robbins 10e Ch 20]