A patient presents with diplopia with limitation of adduction in the left eye and abducting saccades in right eye. Convergence is preserved. Most likely etiology is:

Internuclear ophthalmoplegia (INO) results from damage to the medial longitudinal fasciculus ascending from the abducens nucleus in the pons to the oculomotor nucleus in the midbrain. Damage to fibers carrying the conjugate signal from abducens interneurons to the contralateral medial rectus motoneurons results in a failure of adduction on attempted lateral gaze. For example, a patient with a left INO will have slowed or absent adducting movements of the left eye. Nystagmus is usually present in the abducted eye. Convergence is usually preserved unless the responsible lesion is high in the midbrain. A patient with bilateral injury to the medial longitudinal fasciculus will have bilateral INO. Multiple sclerosis is the most common cause, although tumor, stroke, trauma, or any brainstem process may be responsible. Ref: Hoon J.C. (2012). Chapter 28. Disorders of the Eye. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison's Principles of Internal Medicine, 18e.