What is the expected finding in a patient with cranial nerve III palsy and mydriasis?

• A. Hemorrhagic stroke
• B. Ischemic stroke in the posterior communicating artery
• C. Vascular compression due to tumor
• D. Anterior communicating artery aneurysm

Answer: (B)

In a patient presenting with cranial nerve III palsy accompanied by mydriasis (dilated pupil), the most common cause is ischemia, particularly when there is a vascular compromise from conditions like diabetes or hypertension. The third cranial nerve (oculomotor nerve) is responsible for controlling most of the eye's movements and also parasympathetic innervation that constricts the pupil. In cases of ischemic stroke involving the posterior communicating artery, the resultant damage can disrupt the nerve's pathway, leading to the characteristic findings of ptosis (drooping eyelid), ophthalmoplegia (inability to move the eye), and mydriasis due to loss of parasympathetic control.

Cranial nerve III palsy due to an ischemic event typically presents with these features without additional associated neurologic deficits that would suggest a more widespread issue, which further supports the diagnosis of vascular damage over a compressive issue.

While other options like a hemorrhagic stroke or compression due to a tumor can certainly cause third cranial nerve palsies, the acute presentation generally aligns more clearly with ischemic changes, particularly in the absence of notable additional signs of mass effect or different neurological involvement that would suggest a more non-vascular