A 28-year-old Indian male presents with sudden onset haematuria, hypertension (160/100 mmHg), and mild oedema 2 weeks after a sore throat. Urinalysis shows RBC casts and dysmorphic RBCs. What is the most common cause of acute nephritic syndrome in this clinical scenario?

Explanation

## Post-Streptococcal Glomerulonephritis: The Most Common Cause ### Clinical Presentation & Epidemiology **Key Point:** Post-streptococcal glomerulonephritis (PSGN) is the most common cause of acute nephritic syndrome worldwide, particularly in children and young adults in India and developing nations. **High-Yield:** The classic triad is: 1. Preceding streptococcal infection (pharyngitis or skin infection) — 1–3 weeks latency 2. Sudden onset haematuria with RBC casts (dysmorphic RBCs) 3. Hypertension and mild oedema ### Pathophysiology **Key Point:** Immune complex-mediated glomerulonephritis triggered by nephritogenic strains of *Streptococcus pyogenes* (M types 1, 12, 49). Immune complexes deposit in the glomeruli, activating complement and causing glomerular injury. ### Diagnostic Features | Feature | PSGN | | --- | --- | | **Onset** | Abrupt, 1–3 weeks post-infection | | **Urine findings** | RBC casts, dysmorphic RBCs, mild proteinuria | | **Serum creatinine** | Mildly elevated | | **Complement (C3)** | Depressed (characteristic) | | **ASO titre** | Elevated (confirms streptococcal infection) | | **Kidney biopsy** | Subepithelial "humps" on electron microscopy (pathognomonic) | ### Prognosis **Clinical Pearl:** Most children recover completely with supportive care (fluid restriction, antihypertensives, diuretics). Adults may have slower recovery but >90% achieve normal renal function. Antibiotic therapy (penicillin) does not alter the course once immune complexes have formed but prevents spread of infection. **High-Yield:** Persistent proteinuria, elevated creatinine at presentation, and older age are adverse prognostic factors.