In polyarteritis nodosa, which organ is classically spared from involvement?

• A. Kidneys
• B. Brain
• C. Lungs
• D. Heart

Answer: (C)

Polyarteritis nodosa involves transmural necrotizing inflammation of medium-sized arteries in various organs, but the lungs are classically spared. This pattern helps distinguish PAN from other vasculitides that involve the pulmonary circulation. In PAN, kidneys are often affected (renal artery involvement can cause hypertension and renal microaneurysms without glomerulonephritis), and other organs such as the brain, heart, and gastrointestinal tract can have ischemic or aneurysmal complications. In contrast, diseases like granulomatosis with polyangiitis or microscopic polyangiitis frequently involve the lungs, causing pulmonary symptoms or hemorrhage, which is not typical of PAN. So the lungs being spared is the hallmark in this context.