**Question:** An angiographic study of the distal posterior cerebral artery (PCA) of the Circle of Willis requires access to the vertebral artery. Access to this artery can be obtained from which of the following structures?

A. Internal carotid artery (ICA)
B. Vertebral artery (VA)
C. Posterior communicating artery (PComA)
D. A1 segment of the anterior cerebral artery (ACA)

**Correct Answer:**

**Core Concept:** The Circle of Willis is a circle of large arteries in the brain that supplies blood to the brain. It consists of the anterior cerebral artery (ACA), middle cerebral artery (MCA), posterior cerebral artery (PCA), and the anterior communicating artery (AComA), posterior communicating artery (PComA), and the internal carotid artery (ICA) and vertebral artery (VA) of the circle of Willis.

**Why the Correct Answer is Right:** To perform an angiographic study of the distal posterior cerebral artery (PCA), one needs to access the vertebral artery (VA). The VA originates from the aortic arch and enters the skull to supply blood to the posterior circulation. In this case, the access to VA is essential for the angiographic study.

**Why Each Wrong Option is Incorrect:**

A. Internal carotid artery (ICA) is incorrect because it supplies blood to the anterior circulation, not the posterior circulation. The PCA lies distal to the ICA and relies on the VA for its blood supply.

B. A1 segment of the anterior cerebral artery (ACA) is incorrect because it is a part of the anterior circulation, not the posterior circulation. The PCA originates from the VA and branches off from the PComA.

C. Posterior communicating artery (PComA) is incorrect because it is a part of the circle of Willis, connecting the VA and ACA. The PCA is distal to the PComA and relies on the VA for its blood supply.

D. The correct answer (VA) is the only option that provides access to the vertebral artery, which supplies blood to the posterior circulation and connects with the PCA to form the PCA.

**Clinical Pearl:** Understanding the arterial supply to the brain is crucial for interpreting angiographic studies accurately. The vertebral artery is essential for accessing the posterior circulation, including the PCA, and thus, VA is the correct answer to this question.