The Circle of Willis receives direct input primarily from which vessels?

• A. Internal carotid arteries
• B. Vertebral arteries
• C. Anterior cerebral arteries
• D. Posterior communicating arteries

Answer: (A)

The Circle of Willis is primarily fed by the internal carotid arteries. These two vessels supply the anterior portion of the circle and give rise to the anterior cerebral arteries, while also contributing via the posterior communicating arteries to connect with the posterior circulation. The vertebral arteries contribute to the posterior circulation through the basilar and posterior cerebral arteries, but the main direct input into the circle comes from the internal carotids. Options that describe branches or connections (like the anterior cerebral arteries or posterior communicating arteries) don’t represent a primary source of inflow themselves, and the vertebral arteries feed the circle indirectly through the posterior circulation.