A patient with a lesion of the right oculomotor nerve (CN III) in the interpeduncular fossa presents with complete ptosis and a fixed dilated pupil. The eye is also deviated outward and downward. Which of the following muscles is SPARED?

• A Medial rectus and inferior oblique
• B Superior rectus and inferior rectus
• C Levator palpebrae superioris and inferior oblique
• D Lateral rectus and superior oblique ✓

Explanation

CN III supplies all extraocular muscles except lateral rectus (CN VI) and superior oblique (CN IV). With complete CN III palsy, the eye is pulled out by the intact lateral rectus (CN VI) and down by the superior oblique (CN IV), producing the classic 'down and out' position. The lateral rectus and superior oblique are innervated by CN VI and CN IV respectively, so they are spared. The pupillary constrictor (sphincter pupillae) and ciliary muscle are also CN III-mediated, hence the fixed dilated pupil.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.