**Core Concept**
Proximal Convoluted Tubule (PCT) is a critical segment of the nephron where various ions and substances are reabsorbed or secreted to maintain electrolyte and acid-base balance. The PCT reabsorbs essential nutrients, ions, and water, with the help of various transport mechanisms.

**Why the Correct Answer is Right**
Hydrogen ions (H+) are not absorbed in the PCT. Instead, they are secreted into the lumen to facilitate the reabsorption of bicarbonate ions (HCO3-). The secreted hydrogen ions combine with bicarbonate ions to form carbonic acid, which then dissociates into water and carbon dioxide. This process is mediated by the enzyme carbonic anhydrase, which is present in the brush border and cytoplasm of PCT cells. The reabsorbed bicarbonate ions are then transported back into the bloodstream.

**Why Each Wrong Option is Incorrect**
* **Option A:** Bicarbonate is reabsorbed in the PCT through the secreted hydrogen ions, as explained above.
* **Option B:** Sodium ions (Na+) are reabsorbed in the PCT through the sodium-potassium pump, which maintains sodium balance and helps regulate fluid reabsorption.
* **Option C:** Phosphate ions (PO42-) are also reabsorbed in the PCT through various transport mechanisms, including sodium-phosphate cotransport.

**Clinical Pearl / High-Yield Fact**
The PCT is responsible for reabsorbing approximately 70-80% of filtered bicarbonate, which is essential for maintaining acid-base balance in the body. Impaired bicarbonate reabsorption in the PCT can lead to hyperchloremic metabolic acidosis.

**✓ Correct Answer: D. H+ Hydrogen ions are not absorbed in the Proximal Convoluted Tubule (PCT).**