Rupture of which artery leads to a third nerve palsy with pupillary involvement?

• A. Posterior communicating artery
• B. Anterior cerebral artery
• C. Middle cerebral artery
• D. Basilar artery

Answer: (A)

When a third nerve palsy includes a pupil that is blown (dilated), think a compressive lesion affecting the oculomotor nerve, particularly where the parasympathetic fibers run on its surface. An aneurysm of the posterior communicating artery at the junction with the internal carotid is a classic culprit. As it enlarges, it presses on CN III, leading to the eye being "down and out," ptosis, and a dilated pupil due to loss of pupil constriction.

This pattern helps distinguish compressive causes from microvascular (ischemic) ones, such as diabetes, where the central motor fibers are involved but the superficial parasympathetic fibers—and thus the pupil—are usually spared.

Aneurysms of the anterior cerebral, middle cerebral, or basilar arteries don’t classically produce a third nerve palsy with pupil involvement, because they don’t compress CN III in the same characteristic way.