A 5-year-old boy with bilateral profound congenital hearing loss, white forelock, heterochromia irides, and dystopia canthorum (W index 2.35) is evaluated. Genetic testing confirms PAX3 mutation consistent with Waardenburg Syndrome Type 1. Pure tone audiometry shows air conduction thresholds >90 dB HL across all frequencies. The finding marked **D** in the audiogram—absence of air-bone gap with bone conduction overlaying air conduction—indicates which type of hearing loss pathology?