Damage to the medial longitudinal fasciculus (MLF) on the left side causes which characteristic eye movement disorder?

• A Left eye fails to adduct on leftward gaze; right eye nystagmus on rightward gaze
• B Bilateral upgaze palsy
• C Left eye fails to adduct on rightward gaze; right eye nystagmus on rightward gaze ✓
• D Left eye fails to abduct on rightward gaze

Explanation

The MLF connects the contralateral abducens nucleus (for horizontal gaze) to the ipsilateral oculomotor nucleus (medial rectus subnucleus). A left MLF lesion disconnects the right abducens signal from the left oculomotor; thus, on rightward gaze commands, the right eye abducts normally (with nystagmus), but the left eye fails to adduct — left internuclear ophthalmoplegia (INO). Convergence is typically preserved because it uses a different pathway. Bilateral upgaze palsy and abduction failure indicate different localizations.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.