A 68-year-old woman from Mumbai presents with sudden onset unilateral hearing loss in the left ear over 48 hours, accompanied by vertigo and tinnitus. She denies ear discharge or trauma. Otoscopy is normal. Weber test lateralizes to the right ear. Rinne test shows air conduction better than bone conduction on the left. Pure tone audiometry reveals a 50 dB hearing loss at 4 kHz with no air-bone gap. Tympanometry is normal. What is the most likely diagnosis?