**Core Concept**
The proximal convoluted tubule (PCT) is a critical segment of the nephron where reabsorption of essential ions, glucose, and amino acids occurs. The PCT is also involved in acid-base homeostasis through the exchange of hydrogen ions (H+) for other ions.

**Why the Correct Answer is Right**
In the PCT, H+ ions are exchanged for sodium ions (Na+) through the action of the sodium-hydrogen exchanger (NHE3) enzyme. This process is crucial for maintaining the acid-base balance in the body. The NHE3 enzyme is a type of antiporter that exchanges one Na+ ion for one H+ ion, allowing for the reabsorption of sodium ions and the excretion of excess H+ ions. This exchange also helps to maintain the electrical neutrality of the PCT.

**Why Each Wrong Option is Incorrect**
**Option A:** **Potassium (K+)** - While K+ ions are also exchanged in the PCT, it is not the primary exchange for H+ ions.
**Option B:** **Calcium (Ca2+)** - Ca2+ ions are not exchanged for H+ ions in the PCT.
**Option C:** **Magnesium (Mg2+)** - Mg2+ ions are not the primary exchange for H+ ions in the PCT.

**Clinical Pearl / High-Yield Fact**
The sodium-hydrogen exchanger (NHE3) enzyme plays a critical role in acid-base homeostasis in the PCT. Inhibition of NHE3 can lead to increased excretion of sodium ions and decreased reabsorption of glucose and amino acids.

**Correct Answer:** D.