The circle of Willis (circulus arteriosus) provides collateral circulation between the anterior and posterior cerebral circulations. Which of its components is most commonly absent or hypoplastic as a normal variant, making the posterior cerebral artery territory vulnerable to anterior circulation occlusion?

• A Anterior communicating artery
• B Posterior cerebral artery
• C Posterior communicating artery ✓
• D Anterior cerebral artery A1 segment

Explanation

The posterior communicating artery (PComm) connects the internal carotid artery (anterior circulation) to the posterior cerebral artery (posterior circulation, basilar system). It is the most frequently hypoplastic or absent component of the circle of Willis in anatomical and angiographic studies. When the PComm is hypoplastic or absent, occlusion of the basilar or posterior cerebral artery cannot be collateralised from the anterior circulation. The anterior communicating artery (AComm) is the most common site of berry aneurysm and is less frequently absent than the PComm. A hypoplastic A1 segment is less common.

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

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