A pupil with CN III palsy is most concerning for an aneurysm at the junction where which two arteries meet?

• A. Anterior Communicating Artery and Middle Cerebral Artery
• B. Posterior Communicating Artery and Internal Carotid Artery
• C. Internal Carotid Artery and Basilar Artery
• D. Basilar Artery and Posterior Cerebral Artery

Answer: (B)

Compression of the oculomotor nerve by an aneurysm most often occurs at the junction of the posterior communicating artery with the internal carotid artery. This site is right where CN III travels in the subarachnoid space, so an enlarging aneurysm here can press on the nerve and produce a third-nerve palsy. A hallmark of a compressive CN III palsy is pupil involvement, because the parasympathetic fibers that constrict the pupil lie on the surface of CN III and are affected by the pressure. That's why this junction is the most concerning location when CN III palsy is present.