## Distinguishing PSGN from Post-Skin Infection GN ### Key Clinical Timeline Difference **Key Point:** The latency period between infection and nephritis onset is the most reliable clinical discriminator between PSGN (pharyngitis-associated) and post-infectious GN from skin sources. ### Comparison Table | Feature | PSGN (Pharyngitis) | Post-Skin Infection GN | | --- | --- | --- | | **Latency period** | 7–14 days (range 1–3 weeks) | 1–7 days (often immediate) | | **Infection source** | Group A Streptococcus pharyngitis | Skin infection (impetigo, abscess) | | **Proteinuria** | Mild to moderate (0.5–2 g/day) | Mild to moderate (0.5–2 g/day) | | **RBC casts** | Present (active sediment) | Present (active sediment) | | **Hypertension** | Common (50–60%) | Common (50–60%) | | **Serum complement** | Low C3 (90%) | Low C3 (variable) | ### Why Latency Period Matters **High-Yield:** PSGN classically presents 1–3 weeks after pharyngitis, whereas post-skin infection GN (often caused by non-M protein strains) typically manifests within 1–7 days. This temporal relationship is pathognomonic and guides clinical diagnosis. **Clinical Pearl:** The longer latency in PSGN reflects the time needed for immune complex formation and glomerular deposition; skin-derived GN has a shorter latency because the antigen is more readily accessible to systemic circulation. ### Why Other Features Are NOT Discriminators - **RBC casts, proteinuria, hypertension:** All present in both conditions with similar frequency and severity. - Both are acute proliferative GN with identical histology (endocapillary proliferation, subepithelial "humps"). **Mnemonic:** **LATENCY = LINEAGE** — PSGN (pharynx) = Longer latency; skin GN = shorter latency. 
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