Review Question - QID 109443

A 76-year-old man is brought to his physician's office by his wife due to progressively worsening hearing loss. The patient reports that he noticed a decrease in his hearing approximately 10 years ago. His wife says that he watches television at an elevated volume and appears to have trouble understanding what is being said to him, especially when there is background noise. He states that he also experiences constant ear ringing and episodes of unsteadiness. On physical examination, the outer ears are normal and otoscopic findings are unremarkable. The patient is unable to repeat the sentence said to him on whisper testing. When a vibrating tuning fork is placed in the middle of the patient's forehead, it is heard equally on both ears. When the vibrating tuning fork is placed by the ear and then on the mastoid process, air conduction is greater than bone conduction. Which of the following structures is most likely impaired in this patient?