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: Unknown
A patient with previous spells of diplopia, ataxia, dysarthria, and dizziness becomes acutely comatose. The most likely cause is: 
Select and explain. 
A. Basilar artery thrombosis, B. Subarachnoid hemorrhage, C. Carotid occlusion, D. Cerebellar hemorrhage
A: Basilar artery thrombosis
Answer: A: Basilar artery thrombosis 
Explanation: Patients with basilar artery stenosis frequently have spells of ischemic brainstem dysfunction before a catastrophic stroke caused by arterial thrombosis. Timely anticoagulation and allowing a higher blood pressure can arrest the progression of this potentially fatal stroke. Acute coma can occur in association with each of the cerebrovascular accidents mentioned in the question except carotid occlusion. Sub­arachnoid hemorrhage causes an acute increase in intracranial pressure that reduces blood flow to the brain. Unilateral cortical infarction does not cause coma, but damage to brain­stem structures via infarction or compression will cause coma.