A 26-year-old woman presents with acute onset severe vertigo, nausea, and vomiting, lasting for the past 12 hours. She is unable to stand unsupported. She reports a recent upper respiratory tract infection. On examination, she has horizontal nystagmus beating away from the affected side, which worsens with gaze towards the direction of the nystagmus. Head impulse test is positive (corrective saccade when head turned rapidly towards affected side). Neurological examination is otherwise normal. What is the most likely diagnosis?