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
The commonest cause of pulmonary embolism is - 
Select and explain. 
A. Thrombosis of leg veins, B. Thrombosis of prostatic veins, C. IVC thrombosis, D. Thrombosis of internal pudendal artery
A: Thrombosis of leg veins
Answer: A: Thrombosis of leg veins 
Explanation: Thrombi in a deep vein of the leg accounts for more than 95% of cases of pulmonary embolism.
The most common source of emboli is DVT of femoropopliteal and iliac veins.
But remember that :

Most common site of DVT is calf veins
(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.