A patient develops internuclear ophthalmoplegia (INO): on attempting right lateral gaze, the right eye abducts normally but the left eye fails to adduct and shows monocular nystagmus of the abducting right eye. The lesion is in the:
- A Left medial longitudinal fasciculus ✓
- B Right abducens nucleus
- C Right oculomotor nucleus
- D Left paramedian pontine reticular formation
Correct answer: A. Left medial longitudinal fasciculus
Explanation
In right gaze, the right abducens nucleus fires; its interneurons cross and ascend in the left MLF to activate the right oculomotor nucleus for left eye adduction. A left MLF lesion interrupts this pathway, so the left medial rectus (adduction) fails on attempted right gaze while the abducting eye shows dissociated nystagmus. The lesion is therefore on the side of the adduction failure — the left MLF. Bilateral INO in a young patient strongly suggests multiple sclerosis.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.