A 28-year-old woman presents with progressive bilateral hearing loss over 3 years. She reports that her hearing worsened during her recent pregnancy. Otoscopy is normal, and tuning fork tests show Rinne negative (bone conduction better than air conduction) and Weber lateralizing to the worse ear. Acoustic reflexes are absent. An audiogram is obtained showing a conductive hearing loss pattern with an air-bone gap. The structure marked **A** in the diagram shows a characteristic depression of bone conduction thresholds at 2 kHz. What is the most likely diagnosis?