**Core Concept**
The I cells of the kidney are a type of intercalated cell found in the collecting ducts, playing a crucial role in acid-base balance and electrolyte regulation. These cells are responsible for secreting or reabsorbing hydrogen ions and bicarbonate ions, thereby influencing the pH of the blood and maintaining electrolyte homeostasis.

**Why the Correct Answer is Right**
The I cells contain a unique enzyme called H+-ATPase, which pumps hydrogen ions into the lumen, generating a proton gradient. This process allows the I cells to secrete hydrogen ions, thereby increasing the acidity of the urine and helping to eliminate excess hydrogen ions from the body. This mechanism is essential for maintaining acid-base balance and preventing acidosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the I cells are not primarily involved in the reabsorption of glucose. While the kidneys do play a role in glucose reabsorption, this is not the primary function of the I cells.
* **Option B:** This option is incorrect because the I cells are not responsible for the reabsorption of sodium ions. While the kidneys do reabsorb sodium ions, this is not the primary function of the I cells.
* **Option C:** This option is incorrect because the I cells are not involved in the regulation of potassium ion levels. While the kidneys do play a role in potassium ion balance, this is not the primary function of the I cells.

**Clinical Pearl / High-Yield Fact**
One important clinical correlation to remember is that I cells are involved in the pathogenesis of certain kidney diseases, such as Bartter syndrome, which is characterized by a defect in the H+-ATPase enzyme.

**Correct Answer: C. Secretion of hydrogen ions**