A 60-year-old man presents with palpable purpura, haemoptysis, and rapidly rising creatinine. Renal biopsy shows pauci-immune necrotising crescentic glomerulonephritis. Serology reveals p-ANCA positivity with antimyeloperoxidase specificity. Chest imaging shows no cavitated nodules and no asthma history is reported. What is the diagnosis?

• A Granulomatosis with polyangiitis
• B IgA vasculitis
• C Polyarteritis nodosa
• D Microscopic polyangiitis ✓

Explanation

Microscopic polyangiitis is a pauci-immune necrotising small-vessel vasculitis characterised by MPO-ANCA (p-ANCA) positivity, pulmonary capillaritis causing haemoptysis, and necrotising crescentic glomerulonephritis. It is distinguished from granulomatosis with polyangiitis by the absence of granulomatous inflammation of the upper airway and absence of PR3-ANCA (c-ANCA) and cavitated nodules. Polyarteritis nodosa involves medium-sized vessels, is typically ANCA negative, and does not cause glomerulonephritis, which excludes option C.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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