A 68-year-old man presents with progressive hearing loss over the past 10 years. Audiometry shows symmetric sensorineural hearing loss with the greatest decrement in the 2–8 kHz range, sparing lower frequencies. The structure marked **A** in the diagram (organ of Corti on the basilar membrane) shows tonotopic organization. Which region of the structure marked **A** is most likely affected in this patient's condition?