## **Core Concept**
The question tests understanding of renal physiology, specifically the handling of ions in the renal tubules. In a healthy individual on a normal diet, the kidneys regulate electrolyte balance by adjusting the absorption and excretion of various ions.

## **Why the Correct Answer is Right**
Phosphate ions are almost completely reabsorbed in the renal tubules under normal conditions. This reabsorption primarily occurs in the proximal convoluted tubule (PCT) and is crucial for maintaining phosphate homeostasis. The reabsorption of phosphate is influenced by parathyroid hormone (PTH), which can decrease phosphate reabsorption when its levels are high.

## **Why Each Wrong Option is Incorrect**
* **Option A:** Sodium (Na+) is not completely absorbed; its reabsorption varies along different parts of the nephron and is tightly regulated to maintain sodium balance and, consequently, fluid balance and blood pressure.
* **Option B:** Potassium (K+) reabsorption does occur in certain parts of the nephron, like the proximal tubule, but it is also significantly secreted in the distal parts, especially in the collecting ducts, to maintain potassium balance.
* **Option D:** Calcium (Ca2+) reabsorption is significant but not complete. It occurs mainly in the PCT and the distal convoluted tubule, with PTH playing a key role in regulating its reabsorption.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the kidney's ability to regulate electrolytes is critical for maintaining homeostasis. For phosphate, its complete reabsorption under normal conditions highlights the kidney's role in bone health and mineral metabolism.

## **Correct Answer:** .