## Granulomatosis with Polyangiitis (GPA) — Organ Involvement **Key Point:** GPA is a necrotizing vasculitis affecting small and medium vessels, classically presenting with a triad of upper respiratory tract, lung, and kidney involvement. ### Classic Triad of GPA | Organ System | Frequency | Clinical Features | |---|---|---| | **Upper respiratory tract** | ~90% | Sinusitis, nasal crusting, septal perforation, epistaxis | | **Lower respiratory tract (lungs)** | ~85% | Pulmonary nodules, infiltrates, hemoptysis | | **Kidneys** | ~75% | ANCA-associated glomerulonephritis, rapidly progressive renal failure | | Heart | ~10% | Coronary vasculitis, myocarditis, pericarditis | | GI tract | <5% | Rare; mesenteric vasculitis if involved | **High-Yield:** The **upper and lower respiratory tract** is the MOST commonly affected organ system in GPA, present in the majority of patients at presentation. This distinguishes GPA from microscopic polyangiitis (MPA), which rarely involves the respiratory tract. **Clinical Pearl:** A patient presenting with the triad of sinusitis + pulmonary infiltrates + hematuria should immediately raise suspicion for GPA. Check c-ANCA/PR3 antibodies. **Mnemonic:** **GPA = Granulomas + Pulmonary + ANCA** — the granulomatous inflammation is what makes GPA unique among ANCA-associated vasculitides, and it occurs predominantly in the respiratory tract and kidneys.
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