Okay, let's tackle this question. The patient is 70 years old with dizziness, headache, left-sided hemiparesis, and right eye symptoms like dilatation and ptosis. So, I need to figure out which blood vessel is affected here.

First, the symptoms point to a stroke. Hemiparesis suggests a lesion in the brain affecting motor pathways. Left hemiparesis would mean the right hemisphere is involved since motor pathways cross over. But the eye symptoms are on the right side. Ptosis and dilated pupil (mydriasis) indicate Horner's syndrome. That's a clue because Horner's syndrome is due to disruption of the sympathetic pathway. The right eye symptoms mean the lesion is on the right side.

So, combining Horner's with left hemiparesis. The classic scenario here is a stroke in the posterior circulation, like the vertebral or basilar arteries. The posterior inferior cerebellar artery (PICA) is a branch of the vertebral artery. A PICA stroke can cause lateral medullary syndrome (Wallenberg syndrome). Let me recall the symptoms of Wallenberg: vertebrobasilar insufficiency, ipsilateral Horner's, contralateral loss of pain and temperature, and other features like ataxia. Wait, here the patient has Horner's on the right and left hemiparesis. So, if the lesion is in the right lateral medulla, the Horner's is ipsilateral (right) and the hemiparesis is contralateral (left) because the corticospinal tract is affected. That fits with Wallenberg syndrome. The PICA supplies the lateral medulla, so a PICA aneurysm or thrombosis could be the cause. Alternatively, vertebral artery dissection might also present this way. The other options like anterior cerebral artery (ACA) or middle cerebral artery (MCA) don't fit because they don't involve the sympathetic pathway. The basilar artery is a larger vessel, but specific to the lateral medulla, PICA is more likely here.

Let me check the options. The correct answer is PICA. The other options: if there's an option for vertebral artery, that's possible, but since PICA is a branch, the question is asking for the specific vessel. So the most probable is PICA. The Horner's syndrome is a key differentiator here, pointing to the lateral medulla. So the answer must be PICA.

**Core Concept** The question assesses knowledge of cerebrovascular anatomy, specifically posterior circulation territories and their associated neurological deficits. Key concepts include the posterior inferior cerebellar artery (PICA) and its role in lateral medullary syndrome (Wallenberg syndrome), which combines brainstem and sympathetic pathway dysfunction.

**Why the Correct Answer is Right** The clinical presentation of left hemiparesis (contralateral to a right medullary lesion) and right Horner’s syndrome (ipsilateral sympathetic disruption) is classic for lateral medullary syndrome. This syndrome results from occlusion of the **posterior inferior cerebellar artery (PICA)**, which supplies the lateral medulla. The PICA territory includes the spinothalamic tract (causing contralateral sensory loss), vestibular nuclei (dizziness), and the sympathetic pathway (ipsilateral Horner’s), all consistent with the

✓ Correct Answer: C. Posterior cerebral artery