## Core Concept
The renal tubules play a crucial role in the regulation of electrolytes and water balance in the body. The absorption of ions such as sodium (Na+), potassium (K+), calcium (Ca2+), and magnesium (Mg2+) varies along different segments of the renal tubules.

## Why the Correct Answer is Right
The correct answer, **Magnesium (Mg2+)**, is least absorbed in the tubules because approximately 25% of filtered magnesium is reabsorbed in the renal tubules, primarily in the thick ascending limb of the loop of Henle. This is in contrast to sodium, which is almost completely reabsorbed, and calcium and potassium, which have significant reabsorption as well.

## Why Each Wrong Option is Incorrect
* **Option A: Sodium (Na+)** is extensively reabsorbed throughout the renal tubules, with about 99% of the filtered sodium being reabsorbed by the end of the distal tubule.
* **Option B: Calcium (Ca2+)** is significantly reabsorbed, with about 99% of filtered calcium being reabsorbed in the renal tubules, primarily in the proximal convoluted tubule and the distal convoluted tubule.
* **Option D: Potassium (K+)** has variable handling; while some potassium is reabsorbed, especially in the proximal tubule, the distal tubules and collecting ducts also secrete potassium.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the loop diuretics act on the thick ascending limb of the loop of Henle, where they inhibit the reabsorption of sodium, potassium, and **magnesium**. This results in increased excretion of these ions, which is clinically significant in managing conditions like hypertension and edema.

## Correct Answer: C. Magnesium (Mg2+).