**Core Concept**
Tolvaptan is a selective vasopressin V2 receptor antagonist, which means it works by blocking the action of vasopressin (also known as antidiuretic hormone, ADH) at the V2 receptors in the kidneys. This leads to increased water excretion and decreased water reabsorption, resulting in a mild diuretic effect.

**Why the Correct Answer is Right**
The correct answer, **Low Na (D)**, makes sense because tolvaptan is specifically approved for use in patients with euvolemic or hypervolemic hyponatremia, which is characterized by low sodium levels in the blood. By blocking vasopressin's action, tolvaptan increases the excretion of free water, allowing for the correction of low sodium levels.

Tolvaptan works by inhibiting the V2 receptors in the collecting ducts of the kidneys, which are responsible for water reabsorption. This results in increased urine production and a decrease in water reabsorption, leading to an increase in sodium levels in the blood.

**Why Each Wrong Option is Incorrect**

**Option A:** High Na is incorrect because tolvaptan is not used to treat high sodium levels; instead, it is used to treat low sodium levels.

**Option B:** High K is incorrect because tolvaptan's mechanism of action does not directly affect potassium levels in the blood. It is not used to treat hyperkalemia.

**Option C:** High Ca is incorrect because tolvaptan's mechanism of action does not directly affect calcium levels in the blood. It is not used to treat hypercalcemia.

**Clinical Pearl / High-Yield Fact**
Tolvaptan's approval for use in low sodium levels is a classic exam trap, as it is often confused with other diuretics that are used to treat high sodium levels. Remember that tolvaptan is specifically approved for euvolemic or hypervolemic hyponatremia, making it a high-yield fact for exams.

**✓ Correct Answer: D. Low Na**