A 28-year-old man presents with sudden-onset hematuria, hypertension, and mild renal dysfunction 10 days after a throat infection. Serum complement C3 levels are low. A renal biopsy shows subepithelial 'humps' on electron microscopy. Which feature best distinguishes post-streptococcal glomerulonephritis (PSGN) from IgA nephropathy in this clinical context?

Explanation

## Distinguishing PSGN from IgA Nephropathy ### Electron Microscopy Findings **Key Point:** Subepithelial electron-dense deposits ('humps') are pathognomonic for post-streptococcal glomerulonephritis and are absent in IgA nephropathy. ### Comparison Table | Feature | PSGN | IgA Nephropathy | |---------|------|------------------| | **EM deposits** | Subepithelial 'humps' (pathognomonic) | Mesangial, intramembranous, subendothelial | | **IF pattern** | IgG, C3 (granular) | IgA dominant (mesangial) | | **C3 levels** | Low (transient) | Normal | | **Onset** | Post-infectious (2–3 weeks) | Insidious, often asymptomatic | | **Prognosis** | Usually self-limited | Progressive in 20–30% | ### Why This Matters **High-Yield:** The 'hump' appearance on electron microscopy is virtually diagnostic of PSGN and does not occur in IgA nephropathy. This finding, combined with low C3 and post-streptococcal history, clinches PSGN. **Clinical Pearl:** PSGN typically resolves spontaneously within 6–8 weeks, whereas IgA nephropathy follows a chronic course with progressive renal decline in a subset of patients. ### Why Other Options Are Incorrect - **Option 0 (IgA deposits):** IgA deposits are the hallmark of IgA nephropathy, NOT PSGN. PSGN shows IgG and C3 predominance. - **Option 2 (Hematuria):** Both PSGN and IgA nephropathy present with hematuria; this is not discriminatory. - **Option 3 (Demographics):** Both conditions can present in young males; age and sex are not distinguishing features.