DFNA9 Late-Onset SNHL + Vestibulopathy (COCH)

A 42-year-old man presents with a 3-year history of progressive bilateral hearing loss and recurrent episodes of vertigo with imbalance in the dark. Audiometry shows bilateral high-frequency sensorineural hearing loss with downsloping pattern. Caloric irrigation reveals bilateral reduced responses. His father had similar symptoms starting in his 40s. The clinical presentation marked as **A** in the diagram — progressive high-frequency SNHL with bilateral vestibulopathy — is most consistent with which genetic disorder?