Meniere's Disease Fluctuating Low-Frequency Endolymphatic Hydrops

A 45-year-old woman with a 6-month history of recurrent vertigo episodes (20 minutes to several hours), low-pitched roaring tinnitus, aural fullness, and fluctuating hearing loss in the left ear presents for evaluation. She has experienced two sudden drop attacks. Otoscopy is normal. Weber test lateralizes to the right ear during symptomatic periods; Rinne is positive bilaterally. Pure-tone audiometry between attacks demonstrates the audiometric pattern marked **B** in the diagram. Tympanograms are type A bilaterally. Electrocochleography shows an elevated SP/AP ratio (>0.4). Delayed post-contrast MRI with intratympanic gadolinium confirms endolymphatic hydrops. Which of the following best explains the pathophysiology underlying the audiometric pattern marked **B**?