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

Patients with basilar aery stenosis frequently have spells of ischemic brainstem dysfunction before a catastrophic stroke caused by aerial 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 coical infarction does not cause coma, but damage to brainstem structures infarction or compression will cause coma. Ref - Harrison's principles of internal medicine 20e pg 2073