**Question:** In a healthy individual on a normal diet, which of the following ions is completely absorbed in tubules?

A. Sodium
B. Calcium
C. Chloride
D. Potassium

**Core Concept:** The nephron, the functional unit of the kidney, consists of two main parts: the glomerulus and the tubules. The tubules are responsible for reabsorption of essential ions and molecules from the filtrate back into the bloodstream.

**Why the Correct Answer is Right:**

In a healthy individual on a normal diet, the primary ions absorbed in tubules are:

1. **Sodium (Option D):** Sodium is essential for maintaining osmotic pressure, fluid balance, and nerve impulse transmission. In the nephron, sodium reabsorption occurs mainly in the loop of Henle, distal convoluted tubule, and collecting duct.
2. **Potassium (Option D):** Potassium is also crucial for maintaining osmotic pressure, fluid balance, and nerve impulse transmission. In the nephron, potassium reabsorption occurs primarily in the distal convoluted tubule and collecting duct.

**Why Each Wrong Option is Incorrect:**

3. **Chloride (Option C):** Chloride is mainly reabsorbed in the proximal convoluted tubule and distal convoluted tubule, but not as extensively as sodium and potassium.
4. **Calcium (Option B):** Calcium reabsorption occurs primarily in the distal convoluted tubule and collecting duct. However, compared to sodium and potassium, calcium reabsorption is less significant in maintaining homeostasis.

**Clinical Pearl:** Absorption of sodium and potassium ions is crucial for maintaining osmotic pressure, fluid balance, and nerve impulse transmission. Overly high or low levels can lead to severe medical conditions. Monitoring kidney function and electrolyte balance is essential in assessing overall health and managing various diseases.

**Correct Answer:** D. Potassium and D. Sodium are completely absorbed in tubules, ensuring essential homeostasis and maintaining fluid balance in the body.