The circle of Willis (circulus arteriosus) is formed by anastomoses between which vessels? An aneurysm at the junction of the anterior communicating artery and anterior cerebral artery is particularly associated with which clinical finding if it ruptures?

• A Posterior cerebral arteries connected by the posterior communicating artery to the basilar artery only; rupture causes posterior fossa hemorrhage
• B Bilateral anterior cerebral arteries connected by the anterior communicating artery, and posterior communicating arteries connecting the posterior cerebral to the internal carotid; rupture causes subarachnoid hemorrhage with possible bilateral leg weakness (ACA territory) ✓
• C Middle cerebral arteries connected by anterior and posterior communicating arteries; rupture causes hemiplegia
• D Internal carotid arteries connected directly by the basilar artery; rupture causes brain herniation

Explanation

The circle of Willis is formed by the two anterior cerebral arteries (ACAs) connected by the anterior communicating artery (ACoA), the two internal carotid arteries, the posterior communicating arteries (connecting ICA to posterior cerebral arteries), and the two posterior cerebral arteries. An ACoA aneurysm is the most common intracranial aneurysm (30–40%) and classically ruptures to cause subarachnoid hemorrhage with frontal headache. Rupture may also cause bilateral lower limb weakness (ACA territory), abulia, and memory impairment (bilateral basal forebrain ischemia).

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

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