A 32-year-old woman presents with slowly progressive bilateral hearing loss over 3 years. Otoscopy is normal. Pure tone audiometry shows bilateral conductive hearing loss with an air-bone gap greatest at low frequencies. The audiogram demonstrates a characteristic depression of bone conduction threshold at 2000 Hz (marked as **A** in the diagram). Tympanometry shows type AS pattern with absent acoustic reflexes. High-resolution CT temporal bone confirms otosclerotic foci at the fissula ante fenestram with stapes footplate fixation. Which of the following best describes the pathophysiological basis of the finding marked **A**?
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