This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Surgery
[{'content': 'The commonest cause of pulmonary embolism is - \nSelect and explain. \nA. Thrombosis of leg veins, B. Thrombosis of prostatic veins, C. IVC thrombosis, D. Thrombosis of internal pudendal artery', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Thrombi in a deep vein of the leg accounts for more than 95% of cases of pulmonary embolism.\nThe most common source of emboli is DVT of femoropopliteal and iliac veins.\nBut remember that :\n\nMost common site of DVT is calf veins\n(Calf veins thrombosis are itself associated with a low risk for embolism, but when the thrombosis progresses proximally to involve the larger veins above knee, then the risk becomes very high.', 'role': 'assistant'}]