A lesion confined to the right medial longitudinal fasciculus (MLF) between the abducens and oculomotor nuclei produces which characteristic eye movement abnormality?
- A Right eye fails to abduct on right lateral gaze
- B Right eye fails to adduct on left lateral gaze; left eye has nystagmus on abduction ✓
- C Both eyes fail to look left (conjugate gaze palsy to left)
- D Right eye fails to adduct on right lateral gaze; no nystagmus
Correct answer: B. Right eye fails to adduct on left lateral gaze; left eye has nystagmus on abduction
Explanation
The MLF carries axons from the contralateral abducens interneurons to the medial rectus subnucleus of the ipsilateral oculomotor nucleus, coordinating conjugate horizontal gaze. A right MLF lesion disconnects the left abducens from the right oculomotor nucleus, so on left lateral gaze the right eye fails to adduct while the left eye abducts with nystagmus — this is right internuclear ophthalmoplegia (INO). Abduction failure would suggest an abducens lesion; bilateral gaze palsy suggests a pontine horizontal gaze centre lesion.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
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