An 8-month-old boy with Waardenburg-Shah syndrome (Waardenburg Type 4) due to SOX10 mutation presents with bilateral profound hearing loss. Audiometry shows bilateral thresholds >90 dB HL across all frequencies. The feature marked **D** on the audiogram—absence of air-bone gap with bone conduction overlaying air conduction—confirms which pathophysiological mechanism of hearing loss in this patient?