A patient has paralysis of all extraocular muscles of the left eye, ptosis, and a fixed dilated left pupil but intact corneal reflex. Additionally, left-sided jaw deviation on opening the mouth is absent. Which cranial nerve is the ONLY one affected?

• A Left CN III and CN V3
• B Left CN III ✓
• C Left CN III, CN IV, CN VI
• D Cavernous sinus syndrome sparing CN V1

Explanation

CN III supplies SR, IR, MR, IO (all extraocular muscles except SO and LR), levator palpebrae (ptosis), and the sphincter pupillae via ciliary ganglion (fixed dilated pupil). CN IV supplies SO (intact superior oblique function in CN III palsy is why the eye is deviated 'down and out'); CN VI supplies LR. The description of 'paralysis of all extraocular muscles' can occur with complete CN III palsy when CN IV and VI action moves the eye into specific positions. The corneal reflex requires CN V1 and CN VII, which are intact here. Jaw deviation requires CN V3, which is also intact.

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

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Written and medically reviewed by the StethoPrep medical team.