A 70-year-old woman presents with sudden onset of vertigo, nausea, and vomiting lasting for the past 3 days. She is unsteady on her feet and finds head movements provoke severe dizziness. She has noted some hearing loss in her left ear since this started. Examination reveals nystagmus, positive head impulse test to the left, and sensorineural hearing loss on Rinne's/Weber's tests. What is the most likely diagnosis?