A lesion at the pontomedullary junction involving the medial longitudinal fasciculus (MLF) on the left side would produce which clinical deficit?
- A Right eye cannot adduct on leftward gaze; nystagmus in left abducting eye
- B Bilateral impairment of convergence with normal abduction
- C Left eye cannot adduct on rightward gaze; nystagmus in right abducting eye ✓
- D Left eye cannot abduct on leftward gaze
Correct answer: C. Left eye cannot adduct on rightward gaze; nystagmus in right abducting eye
Explanation
Internuclear ophthalmoplegia (INO) results from MLF lesion. The MLF connects the contralateral abducens nucleus to the ipsilateral oculomotor nucleus. A left MLF lesion disconnects the signal from the right abducens nucleus to the left oculomotor nucleus, so during rightward gaze the left eye fails to adduct while the right eye abducts with nystagmus (ataxic nystagmus). Convergence is typically preserved because it uses a different pathway. In MS, bilateral INO is pathognomonic.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.