**Core Concept**
Tolvaptan is a vasopressin receptor antagonist that 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 a decrease in water reabsorption, resulting in a mild diuresis.

**Why the Correct Answer is Right**
The mechanism of action of tolvaptan involves the blockade of V2 receptors in the renal collecting ducts, preventing the binding of vasopressin to these receptors. This leads to a decrease in the amount of water reabsorbed in the kidneys, resulting in a mild diuresis. Tolvaptan is specifically approved for use in patients with euvolemic or hypervolemic hyponatremia, where the sodium levels in the blood are low due to excessive water retention.

**Why Each Wrong Option is Incorrect**

* **Option A:** Tolvaptan is not approved for use in heart failure. While tolvaptan can increase urine output, it does not improve cardiac function or survival in patients with heart failure.
* **Option B:** Tolvaptan is not approved for use in hypertension. While tolvaptan can cause a mild diuresis, it is not indicated for the treatment of hypertension.
* **Option C:** Tolvaptan is not approved for use in nephrotic syndrome. While tolvaptan can increase urine output, it does not improve proteinuria or reduce edema in patients with nephrotic syndrome.

**Clinical Pearl / High-Yield Fact**
Tolvaptan is a valuable treatment option for patients with euvolemic or hypervolemic hyponatremia, particularly those with heart failure or liver cirrhosis, where fluid overload is a significant concern.

**Correct Answer:** C.