A 35-year-old man with no significant past medical history presents with a 2-day history of severe vertigo, nausea, and vomiting. He reports that the room spins around him and he is unable to stand or walk without support. He noticed an upper respiratory tract infection about a week ago. On examination, he has horizontal nystagmus that is unidirectional, fast phase beating away from the affected side, and fatigable. Head impulse test is positive (corrective saccade when head turned towards the affected side). Neurological examination including cerebellar function is otherwise normal. What is the most likely diagnosis?