This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Ocular motility and squint, Subject name: Ophthalmology
A patient presents with diplopia with limitations of adduction in the left eye and abducting saccade in the right eye. Convergence is preserved. Most likely etiology is - 
Select and explain. 
A. Paial 3rd nerve palsy, B. lnternuclear ophthalmoplegia, C. Duane's reactionary syndrome, D. Absence of medial rectus muscle
B: lnternuclear ophthalmoplegia
Answer: B: lnternuclear ophthalmoplegia 
Explanation: Internuclear ophthalmoplegia. In this condition, there is lesion in the medial longitudinal fasiculus (MLF). There is defective action of medial rectus on the side of lesion. Medial rectus causes adduction of the eye on same side. Contraction of right lateral rectus and left medial rectus causes Dextroversion whereas contraction of left lateral rectus and right medial rectus causes Levoversion. Horizontal nystagmus of opposite convergence is normal. Ref; A.K.Khurana;6th Edition; Page no: 355