Saddle Pulmonary Embolism from Deep Vein Thrombosis

A 65-year-old obese woman (BMI 34) undergoes elective right total hip replacement for osteoarthritis. Postoperatively, she receives mechanical prophylaxis but enoxaparin is inadvertently withheld for 48 hours. On postoperative day 4, she collapses with sudden severe dyspnoea, pleuritic chest pain, and cardiovascular arrest. ECG shows S1Q3T3 pattern with right heart strain. Autopsy reveals the structure marked **C** in the diagram—a coiled thromboembolus straddling the main pulmonary bifurcation with propagation into both main pulmonary arteries. Cast-like deep vein thrombus is recovered from the right femoral and popliteal veins. Which of the following best explains the pathophysiological mechanism underlying the formation of the structure marked **C** in this patient?