A 28-year-old man presents with gross hematuria, hypertension, and mild proteinuria 2 weeks after a sore throat. Serum creatinine is 1.2 mg/dL and complement C3 levels are low. Kidney biopsy shows endocapillary proliferation with 'humps' on electron microscopy. What is the most common cause of acute nephritic syndrome globally?

Explanation

## Most Common Cause of Acute Nephritic Syndrome **Key Point:** Post-infectious glomerulonephritis (PIGN), classically post-streptococcal GN (PSGN), is the most common cause of acute nephritic syndrome globally, particularly in children and young adults in developing countries. ### Clinical Features of PSGN | Feature | Details | |---------|----------| | **Latency** | 1–3 weeks after group A Streptococcus throat/skin infection | | **Presentation** | Gross hematuria, hypertension, mild proteinuria, mild renal impairment | | **Serology** | Low C3 (returns to normal in 6–8 weeks); elevated ASO titre | | **Histology** | Endocapillary proliferation, "humps" on EM (immune complex deposits) | | **Prognosis** | Excellent in children; >95% recover renal function | **High-Yield:** The "humps" on electron microscopy are pathognomonic subepithelial immune complex deposits seen in PSGN — a key exam finding. ### Differential Diagnosis of Acute Nephritic Syndrome ```mermaid flowchart TD A[Acute Nephritic Syndrome]:::outcome --> B{Serology?}:::decision B -->|Low C3 + ASO titre| C[Post-streptococcal GN]:::action B -->|Normal C3| D{IgA or ANCA?}:::decision D -->|IgA dominant| E[IgA nephropathy]:::action D -->|ANCA+| F[ANCA-associated vasculitis]:::urgent B -->|ANA+, low C3/C4| G[Lupus nephritis]:::action ``` **Clinical Pearl:** In India and other developing nations, PSGN remains the leading cause of acute GN in children, whereas IgA nephropathy is the most common primary GN overall (but typically presents with asymptomatic hematuria, not acute nephritic syndrome). **Warning:** Do not confuse "most common GN overall" (IgA nephropathy) with "most common acute nephritic presentation" (PSGN). The question specifically asks for acute nephritic syndrome, which tips the answer toward PSGN.