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?

Question

Accepted Answer

A. Base of the cochlea (near oval window), where the basilar membrane is narrow and stiff and detects high frequencies. ## Why Option 1 (Base of the cochlea) is correct

The organ of Corti exhibits tonotopic organization: the BASE of the cochlea (near the oval window) has a narrow, stiff basilar membrane that vibrates maximally at HIGH frequencies (2–8 kHz), while the APEX has a wide, compliant basilar membrane tuned to LOW frequencies. In presbycusis (age-related sensorineural hearing loss), the basal turn is preferentially damaged because it is exposed to the greatest cumulative sound energy over a lifetime. The patient's audiogram showing high-frequency loss (2–8 kHz) with sparing of low frequencies is the classic pattern of presbycusis, indicating pathology in the basal region of the structure marked **A**. [Guyton & Hall 14e Ch 53; Dhingra ENT 7e]

## Why each distractor is wrong

- **Option 2 (Apex)**: The apex detects LOW frequencies. This patient has HIGH-frequency loss, indicating basal pathology, not apical. Apical damage would spare high frequencies and impair low frequencies—the opposite of what is seen here.
- **Option 3 (Middle turn)**: The middle turn is a transitional zone and does not explain the selective high-frequency loss pattern. Presbycusis has a characteristic base-to-apex progression of damage, not middle-turn predominance.
- **Option 4 (Entire cochlea uniformly)**: Presbycusis does NOT affect all frequencies equally. It characteristically spares low frequencies and preferentially damages high frequencies due to the mechanical properties and cumulative noise exposure of the basal turn.

**High-Yield:** Presbycusis = high-frequency loss first (basal cochlea); noise-induced hearing loss = 4 kHz notch; aminoglycoside ototoxicity = basal outer hair cell damage → high-frequency loss.

[cite:Guyton & Hall 14e Ch 53; Dhingra ENT 7e]

Answer

B. Apex of the cochlea, where the basilar membrane is wide and compliant and detects low frequencies

Answer

C. Middle turn of the cochlea, where both high and low frequencies are detected equally

Answer

D. Entire cochlea uniformly, as presbycusis affects all frequencies equally

Explanation

Practice similar questions