A 42-year-old woman presents with sudden onset vertigo, nausea, and vomiting lasting for the past 48 hours. She is unable to stand or walk without support. She reports a recent upper respiratory tract infection. On examination, she has horizontal nystagmus that worsens with gaze towards the left. Head impulse test is positive (abnormal) when turning the head rapidly to the right. Hearing is normal. What is the most likely diagnosis?