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Antiphospholipid syndrome thrombosis — USMLE Step 3 MCQ

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HardInternal Medicine (chronic management)Antiphospholipid syndrome thrombosisUSMLE Step 3

A 46-year-old woman with systemic lupus erythematosus develops pleuritic chest pain and dyspnea. She takes hydroxychloroquine. Blood pressure is 126/72 mm Hg, pulse is 104/min, and oxygen saturation is 95% on room air. ECG shows sinus tachycardia. D-dimer is elevated, and CT pulmonary angiography shows a small pulmonary embolus. Platelet count is 92,000/mm3, aPTT is prolonged, and anticardiolipin IgG is positive on prior testing 14 weeks ago and today. Which of the following is the most likely explanation for this patient's findings?

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Correct answer: A — Antiphospholipid syndrome

Thrombosis plus persistent antiphospholipid antibodies at least 12 weeks apart establishes antiphospholipid syndrome. The prolonged aPTT reflects lupus anticoagulant effect rather than bleeding tendency.

Reference: EULAR/ACR antiphospholipid syndrome classification and management principles