Speech Audiometry — Half-Optimal Discrimination Score (Retrocochlear)

A 52-year-old man presents with 6 months of progressive unilateral hearing loss and right-sided tinnitus. Pure-tone audiometry shows a moderate sensorineural hearing loss with PTA of 45 dB HL on the right. Speech audiometry reveals a Speech Reception Threshold (SRT) of 48 dB HL, which agrees with PTA. However, when monosyllabic phonetically balanced words are presented at 40 dB above SRT, the patient achieves only 32% correct identification. When intensity is further increased, the score paradoxically decreases to 18%. The finding marked **A** in the diagram — severely reduced SDS with high-intensity rollover — is demonstrated. Which of the following is the most appropriate next diagnostic step?