Review Question - QID 215060

A 34-year-old woman presents to the emergency department with double vision and dizziness. She awoke with these symptoms, which are worse during lateral gaze. She denies any headache or eye pain. She had an episode of transient left arm weakness 1 month ago that has completely resolved. Her past medical history includes depression and vitamin D deficiency. She takes fluoxetine and a multivitamin. On presentation, the patient’s temperature is 98.8°F (37.1°C), blood pressure is 116/74 mmHg, pulse is 72/min, and respirations are 16/min. When asked to look to either side, the patient has normal abduction of the ipsilateral eye but the contralateral eye fails to adduct. There is also horizontal nystagmus of the abducting eye. A retinal exam is unremarkable and there is no ptosis. What is the most likely cause of her visual symptoms?