ABR Threshold Testing in Infants

A 6-month-old male infant with a history of prolonged NICU stay, gentamicin and vancomycin exposure for late-onset sepsis, and hyperbilirubinemia requiring phototherapy is referred for diagnostic ABR testing. Frequency-specific tone-burst ABR is performed with electrodes at the high forehead (non-inverting), ipsilateral mastoid (inverting), and contralateral mastoid (ground). The pattern marked **B** in the diagram shows ABR Wave V thresholds of 65–75 dB nHL at 500–4000 Hz tone bursts with normal Wave V morphology at suprathreshold levels. Bone-conduction ABR thresholds match air-conduction thresholds, and DPOAEs are absent bilaterally. Which of the following best describes the pathophysiologic mechanism underlying this ABR pattern?