Otosclerosis with Carhart notch

A 32-year-old woman presents with progressive hearing loss over 5 years, worsened during her recent pregnancy. She reports hearing better in noisy environments and has a family history of similar hearing loss. Otoscopy reveals an intact tympanic membrane with a subtle pinkish hue over the promontory. Tuning fork tests show Rinne negative at 512 Hz on the affected side, and Weber lateralizes to the worse ear. The audiogram shown displays the pattern marked **A**. Which of the following best explains the pathophysiology of this patient's condition?

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**?

A 32-year-old woman presents with bilateral progressive hearing loss over 5 years, worse on the left, that worsened during her recent pregnancy. She reports paracusis willisii (hearing better in noisy environments). Otoscopy is normal with no Schwartze sign. Tuning fork tests show Rinne negative bilaterally (bone conduction > air conduction) at 256 Hz and 512 Hz, with Weber lateralizing to the worse ear. Pure-tone audiometry reveals bilateral conductive hearing loss of 45 dB on the left and 35 dB on the right. The structure marked **A** in the diagram shows a characteristic notch in bone conduction at 2 kHz (Carhart notch). Tympanometry shows type A with absent stapedial reflexes. What is the pathophysiological basis of the Carhart notch at 2 kHz seen in bone conduction in this patient?