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
Commonest cause of subarachnoid haemorrhage is: 
Select and explain. 
A. Rupture of circle of Willis aneurysm, B. Rupture or veebral aery aneurysm, C. Rupture of venecomitants of corpus striatum, D. Rupture of dural sinusis
A: Rupture of circle of Willis aneurysm
Answer: A: Rupture of circle of Willis aneurysm 
Explanation: Answer is A (Rupture of circle of Willis aneurysm): Most common cause of subarachnoid haemorrhage is ruptured saccular aneurysms. Most common site of saccular aneurysms is the anterior circulation on the circle of Willis