A 38-year-old man presents with a 4-year history of progressive bilateral hearing loss affecting his ability to understand women's and children's voices. He has no noise exposure history, no ototoxic medication use, and no head trauma. His father developed similar hearing loss in his mid-40s, and two paternal cousins have progressive adult-onset hearing loss. Pure tone audiometry shows normal thresholds at 250–500 Hz, mild loss at 1 kHz, and progressively worsening loss at higher frequencies, with severe loss at 8 kHz. The region marked **C** in the diagram—severely affected high-frequency thresholds at 4000–8000 Hz creating a steeply downsloping ski-slope pattern—is characteristic of which genetic condition?