The circle of Willis provides collateral circulation to the brain. In a patient with an isolated occlusion of the right internal carotid artery, which part of the circle can provide flow to the right middle cerebral artery territory via anastomosis?
- A Right external carotid artery via the ophthalmic artery only
- B Right vertebral artery directly
- C Left posterior cerebral artery only via the posterior communicating artery of the left side
- D Anterior communicating artery from the left ICA and posterior communicating artery from the right posterior cerebral artery (basilar system) ✓
Explanation
When the right ICA is occluded, collateral flow to the right MCA territory is provided by: (1) the anterior communicating artery (AcomA), which connects the two ACAs — left ICA flow crosses via the left ACA → AcomA → right ACA and then retrograde into the right ICA supraclinoid segment; (2) the right posterior communicating artery (PcomA), which connects the right PCA (basilar/vertebral system) to the right ICA — basilar flow passes via the right PCA → right PcomA → right ICA. The completeness of this collateral support determines whether infarction occurs. The ophthalmic artery (ICA branch) can receive ECA collateral flow, but this is a minor external-to-internal anastomosis rather than major collateral.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.