Purpura Fulminans — SCE Dermatology MCQ

Correct answer: A — Purpura fulminans

The correct answer is A, purpura fulminans. This woman has a myeloproliferative disorder (essential thrombocythaemia), a rapidly expanding purpuric plaque with central necrosis, and a biopsy showing widespread dermal vascular thrombosis with minimal inflammation, which together are the classic triad for purpura fulminans in its idiopathic/non-infectious form. Purpura fulminans is defined by intravascular coagulation causing thrombotic occlusion of small and medium vessels leading to haemorrhagic skin necrosis, and it is recognised to occur outside the classical meningococcal setting in association with prothrombotic haematological disease. The near absence of inflammatory infiltrate on histology is the key feature distinguishing a thrombotic process from a true vasculitis. Rapid progression to necrosis and the underlying prothrombotic state (thrombocythaemia driving microvascular thrombosis) fit this diagnosis specifically, not the alternatives. Why the other options are wrong: C. Warfarin necrosis: occurs in patients starting warfarin (usually with protein C deficiency), typically affecting fatty areas (breast, thighs, buttocks) within days of starting therapy, but there is no history of anticoagulant use here. D. Livedoid vasculopathy: causes chronic, painful ulcers on the lower legs with atrophie blanche and a relapsing course, biopsy shows segmental hyalinising vasculopathy, not the acute, rapidly necrotic breast plaque described. B. Pyoderma gangrenosum: presents with a violaceous undermined ulcer edge and biopsy shows a dense neutrophilic infiltrate, not minimal inflammation with pure thrombosis. E. Calciphylaxis: occurs in advanced renal failure or hyperparathyroidism with vascular calcification on biopsy, and there is no renal disease or calcium/phosphate derangement mentioned. Key point: A rapidly necrotic purpuric plaque with biopsy showing dermal vessel thrombosis but minimal inflammation, in a patient with a prothrombotic haematological disorder, points to purpura fulminans rather than a vasculitic or infective process.

Reference: DermNet NZ, Purpura Fulminans (reviewed by Dr Ian Coulson), 2025, https://dermnetnz.org/topics/purpura-fulminans