A 28-year-old woman presents with progressive bilateral hearing loss over the past 3 years, worse in the left ear. Otoscopy is normal. Weber test lateralizes to the left ear; Rinne test shows bone conduction better than air conduction on the left. Audiometry reveals a characteristic dip at 2 kHz on bone conduction. The structure marked **B** in the diagram (ossicles) is suspected to be involved. Which of the following best explains the pathophysiology and the specific ossicular abnormality in this patient?