Which structure is formed by the joining of the anterior, middle, and posterior cerebral arteries at the base of the brain?

• A. Circle of Vieussens
• B. Corpus Callosum
• C. Cerebral Cortex
• D. Circle of Willis

Answer: (D)

The structure being tested is the arterial circle at the base of the brain that provides collateral circulation. This circle forms by the connections among the major cerebral arteries: the anterior cerebral arteries join via the anterior communicating artery, and the posterior cerebral arteries connect to the internal carotid system through the posterior communicating arteries, with the basilar artery contributing to the posterior aspect. Together, these vessels create a circular anastomosis—the Circle of Willis—that distributes blood to the brain and can reroute flow if a part is occluded.

Other structures described are not the vascular circle at the base of the brain. The Circle of Vieussens relates to coronary circulation rather than cerebral vessels. The Corpus Callosum is a bundle of nerve fibers connecting the brain’s two hemispheres, and the Cerebral Cortex is the brain’s outer gray matter, not a vascular ring.