A 28-year-old man presents with sudden-onset hematuria, hypertension, and mild renal dysfunction 10 days after a sore throat. Serum complement C3 is low, and anti-GBM antibody is negative. Which single feature best distinguishes post-infectious glomerulonephritis (PIGN) from IgA nephropathy in this clinical context?

## Distinguishing PIGN from IgA Nephropathy ### Clinical Context Both post-infectious GN and IgA nephropathy present with acute nephritic syndrome (hematuria, hypertension, reduced GFR). However, the **temporal relationship to infection** is the most reliable discriminator. ### Key Discriminating Feature **Key Point:** Post-infectious GN (especially post-streptococcal) has a **stereotyped latency period of 1–3 weeks after pharyngitis or skin infection**. IgA nephropathy has **no clear preceding infection** and often presents insidiously or with recurrent hematuria unrelated to upper respiratory tract infections. ### Comparison Table | Feature | PIGN (Post-Streptococcal) | IgA Nephropathy | |---------|---------------------------|------------------| | **Preceding infection** | Streptococcal pharyngitis or pyoderma (1–3 weeks prior) | None; or viral URI without clear temporal link | | **Complement C3** | Low (classic finding) | Normal | | **Onset** | Acute, explosive | Insidious or episodic | | **Age** | Children > adults | Young adults; can be any age | | **Prognosis** | Excellent in children; most recover | Variable; 20–30% progress to ESRD | | **Dysmorphic RBCs** | Present | Present | | **Proteinuria** | Mild to moderate | Often present | ### Why This Matters **High-Yield:** The **1–3 week latency after documented streptococcal infection** is pathognomonic for PIGN and is almost never seen in IgA nephropathy. This temporal relationship is the single most specific historical clue. **Clinical Pearl:** A patient who says "I had a sore throat 2 weeks ago, now I have hematuria" → think PIGN. A patient with "recurrent hematuria for months, no clear trigger" → think IgA nephropathy. ### Why Other Features Don't Discriminate - **Dysmorphic RBCs and casts:** Both conditions produce these (glomerular origin of bleeding). - **Elevated creatinine:** Both can present with reduced GFR; not discriminatory. - **Proteinuria:** Both typically have proteinuria; its absence does not distinguish them.