Circle of Willis — FRCA Primary MCQ
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Correct answer: A — Internal carotid arteries and vertebrobasilar system
Explanation lettering: C = shown as A · E = shown as B · D = shown as C · A = shown as D · B = shown as E
C is correct. The circle of Willis links the anterior circulation, supplied by the paired internal carotid arteries, with the posterior circulation, supplied by the vertebrobasilar system. The vertebral arteries unite to form the basilar artery, which terminates as the posterior cerebral arteries; the posterior communicating arteries connect these to the internal carotid circulation. The anterior part of the circle includes the proximal anterior cerebral arteries and anterior communicating artery. The external carotid arteries do not normally contribute directly to the circle, although extracranial–intracranial collateral pathways may become important in disease. The common carotid and subclavian arteries are upstream parent vessels rather than components or direct intracranial contributors.
Reference: Willie CK, Tzeng YC, Fisher JA, Ainslie PNA. Integrative regulation of human brain blood flow, Figure 1. Journal of Physiology. 2014;592(5):841–859. https://pubmed.ncbi.nlm.nih.gov/24396059/