This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
A 26-year-old woman with multiple sclerosis comes to the physician because of a recent relapse Examination of the eye movements shows slow adduction of the left eye with an associated right-beating nystagmus in the right eye. 

Which of the following is the location of the pathology that underlies this abnormality? 
Select and explain. 
A. Abducens nerve palsy, B. Left medial longitudinal fasciculus, C. Right medial longitudinal fasciculus, D. Right oculomotor nerve
B: Left medial longitudinal fasciculus
Answer: B: Left medial longitudinal fasciculus 
Explanation: None