**Core Concept**
A non-selective vasopressin receptor antagonist is a type of medication that blocks all subtypes of vasopressin receptors, including V1A, V1B, and V2 receptors. This action is used to treat conditions such as hyponatremia and heart failure.

**Why the Correct Answer is Right**
Non-selective vasopressin receptor antagonists, like conivaptan and tolvaptan, competitively inhibit vasopressin binding to its receptors. This leads to decreased water reabsorption in the kidneys, increased urine output, and subsequent correction of hyponatremia. Additionally, blockade of V1A receptors results in vasodilation and reduced blood pressure, which is beneficial in heart failure patients.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a non-selective vasopressin receptor antagonist.
**Option B:** This option is not a known medication for treating hyponatremia.
**Option C:** This option is a selective V2 receptor antagonist, used primarily for treating hyponatremia, but not non-selective.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that non-selective vasopressin receptor antagonists can cause hypotension, especially in patients with hypovolemia or those taking other vasodilators.

**Correct Answer: C. Tolvaptan**