A 38-year-old woman presents with sudden onset of severe vertigo, nausea, and vomiting, lasting for the past 6 hours. She reports hearing loss and tinnitus in her right ear. She has had two similar, but less severe, episodes in the past year, each lasting a few hours. Otoscopy is normal. Neurological examination is otherwise unremarkable, with no focal deficits. What is the most likely diagnosis?