A 42-year-old woman presents to the ENT clinic with a 6-month history of episodic vertigo lasting 20–30 minutes, associated with tinnitus and a sensation of fullness in the left ear. During the last episode 2 days ago, she experienced sudden sensorineural hearing loss at low frequencies (500–2000 Hz) on audiometry. Between episodes, hearing partially recovers. She denies recent head trauma or ototoxic drug exposure. Caloric testing shows reduced response on the left. What is the most likely diagnosis?