A patient with previous spells of diplopia, ataxia, dysarthria, and dizziness becomes acutely comatose. The most likely cause is:

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.