**Core Concept**
The renal medulla's hyperosmolarity is a critical physiological adaptation that enables the concentration of urine. This is primarily due to the countercurrent mechanism in the loop of Henle, which creates a concentration gradient.

**Why the Correct Answer is Right**
The renal medulla's hyperosmolarity is mainly attributed to the high concentration of sodium ions (Na+). In the loop of Henle, particularly in the thick ascending limb, the Na+/K+/2Cl- cotransporter (NKCC2) reabsorbs sodium ions, potassium ions, and chloride ions. This reabsorption leads to a significant increase in sodium ions in the medullary interstitium, contributing to its hyperosmolarity. The sodium ions also contribute to the osmotic gradient that drives water reabsorption in the collecting ducts.

**Why Each Wrong Option is Incorrect**
**Option A:** Potassium ions (K+) are indeed present in the renal medulla but do not primarily contribute to its hyperosmolarity. The high concentration of potassium ions is more related to the potassium-sparing effect of aldosterone in the collecting ducts.
**Option C:** Glucose is not a primary contributor to the renal medulla's hyperosmolarity. Although glucose can be reabsorbed in the proximal convoluted tubule, its presence is not significant enough to affect the medulla's osmotic balance.
**Option D:** The notation "Na ++" is incorrect, as sodium ions are typically represented as Na+.

**Clinical Pearl / High-Yield Fact**
The countercurrent mechanism in the loop of Henle is a critical adaptation that enables the kidneys to concentrate urine. This process is essential for maintaining proper fluid balance and electrolyte homeostasis in the body.

**✓ Correct Answer: B. Na +**