The circle of Willis is an anastomotic ring at the base of the brain. Which vessel most commonly causes subarachnoid hemorrhage when it ruptures due to a saccular (berry) aneurysm at the junction of which two arteries?

• A Anterior communicating artery at the junction with anterior cerebral arteries ✓
• B Middle cerebral artery at its M1 trifurcation
• C Posterior communicating artery at its junction with internal carotid artery
• D Basilar artery at its bifurcation

Explanation

The anterior communicating artery (AComm) is the most common site of intracranial berry aneurysm (approximately 35-40% of all intracranial aneurysms), followed by the posterior communicating artery junction with the internal carotid artery (30%). AComm aneurysms classically rupture into the subarachnoid space causing sudden 'thunderclap' headache. Large AComm aneurysms can also compress the optic chiasma (bitemporal hemianopia) or hypothalamus. The 'top of basilar' aneurysm (at the bifurcation into posterior cerebral arteries) causes top-of-basilar syndrome with upgaze palsy and altered consciousness.

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

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Written and medically reviewed by the StethoPrep medical team.