The circle of Willis provides collateral circulation at the base of the brain. The posterior communicating artery connects which two major arterial systems and is a common site for berry aneurysm?

• A Internal carotid artery to posterior cerebral artery (connecting anterior and posterior circulations) ✓
• B Middle cerebral artery to posterior cerebral artery
• C Anterior cerebral artery to basilar artery
• D Anterior communicating artery connects both anterior cerebral arteries

Explanation

The posterior communicating artery runs from the internal carotid artery (anterior circulation) to the posterior cerebral artery (posterior circulation, a branch of the basilar artery), thereby linking the two circulations in the circle of Willis. It is a common site for saccular (berry) aneurysms because of haemodynamic stress at the ICA-PComm junction. Aneurysm at this location presents with CN III palsy (fixed dilated pupil) due to pressure on the oculomotor nerve, often before subarachnoid haemorrhage. Option D describes the anterior communicating artery, the overall most common site for berry aneurysm.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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