A 34-year-old woman with relapsing-remitting multiple sclerosis has horizontal diplopia. On attempted right lateral gaze, the right eye abducts normally but the left eye fails to adduct, and there is abducting nystagmus of the right eye. Convergence is preserved. Where is the lesion?

• A Left medial longitudinal fasciculus ✓
• B Right abducens nucleus
• C Right oculomotor nucleus
• D Left medial rectus muscle

Explanation

Internuclear ophthalmoplegia is caused by a demyelinating lesion of the medial longitudinal fasciculus on the side of the impaired adduction, disconnecting the abducens nucleus from the contralateral medial rectus subnucleus. Preserved convergence confirms the medial rectus itself and its lower motor neuron pathway are intact, excluding a third nerve or muscular cause.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.