The patient's symptoms and findings are highly indicative of a cerebellar stroke.

The patient's symptoms and findings are highly indicative of a cerebellar stroke.

• Sudden severe occipital headache, vertigo, and difficulty walking (ataxia) are classic symptoms of cerebellar dysfunction.
• Nystagmus is a common neurological sign associated with cerebellar or brainstem lesions.
• The most definitive finding is the MRI showing infarction in the cerebellum. An infarction is tissue death due to lack of blood supply, which is the definition of a stroke. When this occurs in the cerebellum, it is specifically called a cerebellar stroke.

The correct option is D.

$$\boxed{\text{D. Cerebellar Stroke}}$$