A 29-year-old woman with relapsing-remitting multiple sclerosis reports horizontal double vision. Examination shows impaired adduction of the right eye on left gaze with abducting nystagmus of the left eye, while convergence is preserved. Where is the lesion?
- A Right abducens nucleus
- B Right medial longitudinal fasciculus ✓
- C Left oculomotor nucleus
- D Right optic radiation
Correct answer: B. Right medial longitudinal fasciculus
Explanation
Ipsilateral adduction failure with contralateral abducting nystagmus and preserved convergence is internuclear ophthalmoplegia, caused by a demyelinating lesion of the medial longitudinal fasciculus on the side of the weak adduction, here the right MLF. An abducens nucleus lesion causes a conjugate gaze palsy, not selective adduction failure. Preserved convergence distinguishes this nuclear third nerve palsy, killing option C.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.