A 7-year-old child from rural India presents with a 6-month history of painless, intermittent mucopurulent ear discharge from the left ear following an episode of acute otitis media. Otoscopy reveals a central perforation of the tympanic membrane with normal middle ear mucosa. Pure-tone audiometry is performed, and the diagram shows the audiometric pattern. The structure marked **A** in the diagram represents the air-bone gap characteristic of conductive hearing loss. Which of the following best describes the PRIMARY pathophysiological basis for this air-bone gap in this patient's condition?