You are answering a USMLE-style medical question. Question: A 25-year-old woman comes to the physician because of periodic painful double vision for 3 days. Her symptoms occur only when looking sideways. She has myopia and has been wearing corrective lenses for 10 years. Ten days ago, she lost her balance and fell off her bike, for which she went to a hospital. A CT scan of the head at that time showed no abnormalities and she was released without further treatment. Her only medication is an oral contraceptive. Her vital signs are within normal limits. The pupils are equal and reactive to light. Her best corrected visual acuity is 20/40 in each eye. She has an adduction deficit in the right eye and nystagmus in the left eye when looking left; she has an adduction deficit in the left eye and nystagmus in the right eye when looking right. Fundoscopy reveals bilateral disc hyperemia. Which of the following is the most likely cause of this patient's findings? Options: A) Demyelination of the medial longitudinal fasciculus B) Compressive tumor of the pons C) Oxidative damage due to thiamine deficiency D) Infarction of the brainstem Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.