## **Core Concept**
The auditory hair cells in the inner ear are crucial for converting sound vibrations into electrical signals that are interpreted by the brain. The deflection of stereocilia on these hair cells leads to changes in the electrical properties of the cell, which can either excite or inhibit the cell depending on the direction of deflection.

## **Why the Correct Answer is Right**
When stereocilia are deflected towards the tallest stereocilium, mechanically gated potassium channels open. This allows positively charged potassium ions (K+) to flow into the cell. The endolymph surrounding the apical surface of hair cells is rich in potassium ions, which is unique compared to most bodily fluids. The influx of potassium ions depolarizes the hair cell. This depolarization opens voltage-gated calcium channels, leading to neurotransmitter release onto the afferent nerve fibers, which then transmit signals to the brain.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not accurately describe the physiological response to stereocilia deflection.
- **Option B:** This option is incorrect because it does not accurately reflect the ionic movement or the physiological response.
- **Option C:** Depolarization of the hair cell leads to an increase in intracellular calcium and subsequent neurotransmitter release, which is not described here.
- **Option D:** This option does not correctly represent the immediate electrical or ionic consequence of stereocilia deflection.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the cochlear endolymph is unique due to its high potassium concentration, which is crucial for the mechanotransduction process in hair cells. Damage to these hair cells or the stereocilia can lead to hearing loss.

## **Correct Answer:** D. Depolarization of the hair cell.