Which artery is described as the single midline artery formed by the fusion of the vertebral arteries at the bottom of the pons?

• A. Vertebral arteries
• B. Basilar artery
• C. Internal carotid arteries
• D. AICA

Answer: (B)

The key idea is that the basilar artery is the single midline vessel formed by the fusion of the two vertebral arteries at the lower part of the brainstem. As the vertebral arteries ascend and come together at the base of the pons, they merge to create the basilar artery, which then runs along the ventral surface of the brainstem to supply the posterior circulation. This differs from the vertebral arteries themselves, which are paired vessels that unite to form that one midline channel. The internal carotid arteries supply the anterior circulation, and the AICA is a branch that comes off the basilar to perfuse the cerebellum, not the vessel formed by the fusion.