This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory system, Subject name: Anatomy
[{'content': 'The most common cause of pulmonary embolism is \nSelect and explain. \nA. Amniotic fluid embolism, B. Calf vein thrombi, C. Pelvic vein thrombosis, D. Cardio thoracic surgery', 'role': 'user'}, {'content': "Answer: C \nExplanation: About half of the patient with pelvic vein thrombosis or proximal leg deep vein thrombosis have pulmonary thromboembolism. Isolated calf vein thrombosis pose a lower risk of pulmonary embolism. The majority of pulmonary emboli arise from the propagation of lower limb deep vein thrombosis (p. 186). Rare causes include septic emboli (from endocarditis affecting the tricuspid or pulmonary valves), tumour (especially choriocarcinoma), fat following fracture of long bones such as the femur, air, and amniotic fluid, which may enter the mother's circulation following delivery. Ref Davidson edition23rd pg 620", 'role': 'assistant'}]