**Core Concept**
The loop of Henle, part of the nephron in the kidney, is crucial for water and ion reabsorption. It consists of two main sections: the descending limb, where water and ions are reabsorbed, and the ascending limb, where ions are reabsorbed against an electrochemical gradient.

**Why the Correct Answer is Right**
Urea is not significantly absorbed in the loop of Henle. Instead, it is passively reabsorbed in the proximal convoluted tubule and the peritubular capillaries, and its reabsorption rate is influenced by osmolality and blood flow. The loop of Henle primarily reabsorbs ions such as sodium (Na+), potassium (K+), and chloride (Cl-), which helps maintain the osmotic gradient necessary for water reabsorption.

**Why Each Wrong Option is Incorrect**
**Option A:** K+ is actively reabsorbed in the ascending limb of the loop of Henle through the Na+, K+-ATPase pump, which helps maintain potassium homeostasis and sodium balance.

**Option C:** Cl- is reabsorbed in the thick ascending limb of the loop of Henle through the Na+-K+-2Cl- cotransporter, which helps regulate the osmotic gradient.

**Option D:** Na+ is reabsorbed in the thick ascending limb of the loop of Henle through the Na+-K+-2Cl- cotransporter, which helps regulate blood pressure and maintain electrolyte balance.

**Clinical Pearl / High-Yield Fact**
The loop of Henle's ion reabsorption plays a crucial role in maintaining the osmotic gradient necessary for water reabsorption, which is essential for regulating blood volume and blood pressure. A defect in the loop of Henle's ion reabsorption can lead to conditions such as Bartter syndrome, characterized by hypokalemia, metabolic alkalosis, and normal or low blood pressure.

✓ Correct Answer: B. Urea