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: Medicine
[{'content': 'The most common source of pulmonary embolism is: \nSelect and explain. \nA. Amniotic fluid embolism, B. Calf vein thrombi, C. Large veins of leg, D. Cardio thoracic surgery', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Answer is C (Large veins of leg): About half the patients with pelvic vein thrombosis or proximal leg deep vein thrombosis have pulmonary thromboembolism. Isolated calf vein thrombosis pose a lower risk of pulmonary embolism- Harrisons', 'role': 'assistant'}]