A 52-year-old woman is evaluated for acute onset of right-sided hearing loss that began yesterday. Soon afterward, she also noted a sensation of ear fullness and ringing in the same ear. She has no other focal neurologic symptoms. She reports no rhinorrhea, fever, pharyngitis, or ear pain. Medical history is significant for hypertension. She takes chlorthalidone. She has had no other exposures to medications or supplements. On physical examination, vital signs are normal. There is decreased hearing in the right ear; the Weber test lateralizes to the left ear, and air conduction is louder than bone conduction bilaterally. The ear canals are unobstructed, and the tympanic membranes are normal appearing. The neurologic examination is unremarkable. What is the most likely diagnosis?