Which arteries form the Circle of Willis and how are they connected?

• A. Internal carotid arteries and posterior cerebral arteries connected via anterior cerebral and posterior communicating arteries; basilar artery provides vertebrobasilar input.
• B. Cerebral veins form a circle that evenly supplies all brain regions.
• C. Anterior cerebral arteries connect directly to vertebral arteries.
• D. Vascular input to the circle comes only from the internal carotid arteries.

Answer: (A)

The main idea is that the Circle of Willis is an arterial ring at the base of the brain that creates a safety net for blood flow, formed by connections between the anterior and posterior circulation. It is built from the internal carotid arteries giving rise to the anterior cerebral arteries, which are joined by the anterior communicating artery. The internal carotids also connect to the posterior cerebral arteries through the posterior communicating arteries, and the posterior cerebral arteries originate from the basilar artery, which brings in the vertebrobasilar input. This arrangement—internal carotids and posterior cerebral arteries linked by the anterior and posterior communicating arteries, with the basilar artery providing the posterior (vertebrobasilar) input—is what forms the Circle of Willis.