A patient with a posterior communicating artery aneurysm develops ptosis, mydriasis, and the eye is directed 'down and out.' Which nerve is compressed, and at which location does the parasympathetic component travel?

• A CN IV; parasympathetic fibers run in the core of the nerve
• B CN III; parasympathetic fibers run deep in the core of the nerve
• C CN VI; no parasympathetic component
• D CN III; parasympathetic fibers run superficially on the periphery of the nerve ✓

Explanation

The oculomotor nerve (CN III) carries preganglionic parasympathetic fibers that travel superficially on the outer surface of the nerve. Extrinsic compressive lesions (e.g., aneurysm, uncal herniation) preferentially damage these peripheral fibers first, producing mydriasis alongside ptosis and external ophthalmoplegia. In contrast, ischemic CN III palsy (e.g., diabetes) spares the pupil because it affects central axons.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.