Final answer:
The right and left vertebral arteries are NOT part of the Circle of Willis, which is a crucial structure at the base of the brain that consists of an anastomosis of several arteries.
Step-by-step explanation:
The option that is NOT part of the Circle of Willis is d) Right and left vertebral arteries. The Circle of Willis is a circular anastomosis of arteries located at the base of the brain. The Circle of Willis is a network of arteries in the brain that helps to ensure constant blood flow to the brain. The arteries that make up the Circle of Willis include the right and left anterior cerebral arteries, the right and left internal carotid arteries, the anterior communicating artery, and the right and left vertebral arteries. The right and left anterior cerebral arteries, internal carotid arteries, and anterior communicating artery form part of the anterior circle, while the posterior part is formed by the posterior communicating arteries coming from the posterior cerebral arteries, which arise from the basilar artery. The vertebral arteries, on the other hand, join to form the basilar artery and are not part of the circle but are essential for its supply.