A 42-year-old man with a 15-year history of chronic ear discharge and progressive hearing loss presents for audiological assessment. Pure-tone audiometry shows bone conduction thresholds elevated to 35 dB across mid-frequencies and an air-bone gap of 25 dB. The structure marked **A** in the diagram represents the pattern seen in this patient. Which of the following best explains the pathophysiology underlying this audiometric finding in chronic suppurative otitis media?