A patient develops internuclear ophthalmoplegia (INO) with adduction failure of the right eye and nystagmus of the abducting left eye on left gaze. The lesion is located in:
- A Right medial longitudinal fasciculus (MLF) ✓
- B Left medial longitudinal fasciculus (MLF)
- C Right oculomotor (CN III) nucleus
- D Left abducens (CN VI) nucleus
Correct answer: A. Right medial longitudinal fasciculus (MLF)
Explanation
Internuclear ophthalmoplegia results from a lesion in the medial longitudinal fasciculus (MLF) ipsilateral to the eye with adduction failure. The right MLF connects the left abducens nucleus to the right oculomotor nucleus; its disruption prevents the right medial rectus from receiving the adduction signal during left conjugate gaze. The abducting left eye shows dissociated nystagmus (ataxic nystagmus). Bilateral INO in young adults is a classic indicator of multiple sclerosis; unilateral INO in older patients is often vascular.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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