What arteries confluence to form the basilar artery?

The basilar artery is formed by the confluence of the left and right vertebral arteries. These arteries ascend through the cervical vertebrae and enter the skull through the foramen magnum, where they merge at the lower border of the pons. The formation of the basilar artery is crucial for supplying blood to the brainstem, cerebellum, and parts of the posterior cerebral circulation.

The vertebral arteries are essentially the vessels that directly branch off the subclavian arteries, providing critical blood flow to the posterior aspects of the brain. This role underlines the importance of the vertebral arteries in ensuring adequate cerebral perfusion. In contrast, the other options listed involve different vascular structures that do not contribute to the formation of the basilar artery. For instance, the internal carotid arteries primarily supply the anterior portions of the brain, while the anterior cerebral arteries branch off the internal carotids but do not converge to form the basilar artery. The posterior communicating arteries form part of the Circle of Willis and connect the internal carotid arteries with the posterior cerebral arteries, but they are not involved in the formation of the basilar artery either.