A 15-year-old boy with congenital bilateral profound sensorineural hearing loss presents with progressive night blindness and tunnel vision. His audiogram shows bilateral profound hearing loss with air conduction thresholds >90 dB HL across all frequencies. The feature marked **D** in the diagram (absence of air-bone gap with bone conduction overlaying air conduction) confirms purely cochlear pathology. Molecular testing reveals biallelic CDH23 mutations. Which of the following best explains why the absence of air-bone gap (marked **D**) is diagnostic of the cochlear involvement in this patient's condition?
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