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
The most common source of pulmonary embolism is: 
Select and explain. 
A. Amniotic fluid embolism, B. Calf vein thrombi, C. Large veins of leg, D. Cardio thoracic surgery
C: Large veins of leg
Answer: C: Large veins of leg 
Explanation: 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