A 40-year-old woman with multiple sclerosis has impaired adduction of the left eye on attempted right gaze, with abducting nystagmus of the right eye, but convergence is intact. Which structure is affected?
- A Left abducens nucleus
- B Left medial longitudinal fasciculus ✓
- C Right medial longitudinal fasciculus
- D Parietal eye field
Correct answer: B. Left medial longitudinal fasciculus
Explanation
Internuclear ophthalmoplegia results from a lesion of the medial longitudinal fasciculus connecting the abducens nucleus to the contralateral medial rectus subnucleus of the oculomotor complex. A left MLF lesion prevents adduction of the left eye on attempted right gaze while sparing convergence, which uses a separate pathway. The abducting nystagmus appears in the opposite (right) abducting eye. An abducens nucleus lesion would produce a conjugate gaze palsy instead.
Reference: Snell's Clinical Neuroanatomy, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.