A 52-year-old woman presents with progressive unilateral hearing loss over 18 months. Pure-tone audiometry shows the pattern marked **A** in the diagram. She reports tinnitus but denies vertigo. Speech discrimination is only 45% at 40 dB HL, despite a pure-tone threshold of 35 dB at 4 kHz. Tympanometry is normal and acoustic reflexes are present bilaterally. Which of the following is the most likely diagnosis?