A 57-year-old banker complains of 2 days of severe dizziness. When she sits up or rolls over in bed, she has a spinning sensation that lasts for a few seconds and is followed by nausea. These episodes have occurred many times a day and prevent her from working. She denies fever or upper respiratory symptoms and takes no medications. When moved quickly from a sitting to a lying position with her head turned and hanging below the horizontal plane, she complains of dizziness, and horizontal nystagmus is noted. The most likely diagnosis is