A 35-year-old woman on oral contraceptives develops sudden onset pleuritic chest pain and dyspnea 10 days after orthopedic surgery. D-dimer is elevated. CT pulmonary angiography confirms bilateral pulmonary emboli. What is the most common source of pulmonary emboli?

• A Right atrial thrombus
• B Pelvic vein thrombosis
• C Deep veins of the lower extremities (iliofemoral system) ✓
• D Subclavian vein thrombosis

Explanation

More than 95% of pulmonary emboli originate from deep vein thrombosis of the lower extremities, particularly the iliofemoral venous system. Virchow's triad of stasis, endothelial injury, and hypercoagulability are the risk factors — this patient has all three (OCP use, post-surgical immobility, and venous injury). Large saddle emboli can cause acute right heart failure, while smaller emboli cause pulmonary infarction in areas with poor collateral circulation, typically at the peripheral pleural base.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

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