A 58-year-old Indian woman presents to the emergency department with acute dyspnea, pleuritic chest pain, and tachycardia (HR 118/min, RR 26/min). She had undergone total abdominal hysterectomy 5 days ago and was immobilized for 3 days post-operatively. CT pulmonary angiography (CTPA) is performed. The structure marked **A** in the diagram shows an intraluminal filling defect in the right main pulmonary artery. Which of the following is the MOST LIKELY source of the thrombus responsible for this imaging finding?

Explanation

## Why Deep vein thrombosis of the iliofemoral veins is right The clinical anchor explicitly states that acute pulmonary embolism results from obstruction of the pulmonary arterial circulation by thrombus, "most often originating from DEEP VEIN THROMBOSIS of the LOWER EXTREMITIES (iliofemoral)." The patient has two major components of Virchow's triad: venous stasis (post-operative immobilization for 3 days) and endothelial injury (surgical trauma from hysterectomy). The intraluminal filling defect marked **A** in the right main pulmonary artery represents the embolized thrombus that has traveled from the deep veins of the lower extremities through the right heart to lodge in the pulmonary circulation. This is the pathophysiologic pathway emphasized in Harrison 21e and Brant & Helms. ## Why each distractor is wrong - **Right atrial thrombus secondary to atrial fibrillation**: While atrial fibrillation can predispose to thrombus formation in the right atrium, the clinical presentation and risk factors (post-operative state, immobilization) point to venous thromboembolism from the lower extremities. Additionally, right atrial thrombi are less common sources of PE compared to DVT of the lower extremities. - **Paradoxical embolism through a patent foramen ovale from a right ventricular thrombus**: Paradoxical embolism is a rare cause of PE and would require both a PFO and a source of thrombus in the right ventricle. The clinical context (post-operative immobilization) and epidemiology strongly favor DVT from the lower extremities as the source. - **Thrombosis of the superior vena cava due to central venous catheter placement**: There is no mention of central venous catheter placement in this patient. While SVC thrombosis can theoretically lead to PE, it is not the most common source and is not supported by the clinical scenario presented. **High-Yield:** Acute PE originates from DVT of the lower extremities in >90% of cases; Virchow's triad (stasis, endothelial injury, hypercoagulability) drives thrombus formation in post-operative patients. [cite: Harrison 21e Ch 277; Brant & Helms Fundamentals of Diagnostic Radiology 5e]