**Core Concept**
The Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is a condition characterized by excessive secretion of antidiuretic hormone (ADH), leading to water retention and hyponatremia. **Vasopressin** receptor antagonists are used to treat SIADH.

**Why the Correct Answer is Right**
The correct answer is a **vasopressin receptor antagonist**, which works by blocking the action of ADH at the level of the kidneys, thereby increasing water excretion and correcting hyponatremia. This class of drugs is specifically designed to target the **V2 receptors** in the renal collecting ducts.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not a vasopressin receptor antagonist.
**Option B:** Incorrect as it does not act on the vasopressin receptors.
**Option C:** Incorrect because it is not used in the treatment of SIADH.

**Why Each Wrong Option is Incorrect (Continued)**
**Option D:** Incorrect, however, the correct answer is actually related to this option, as **Tolvaptan** is a vasopressin receptor antagonist used in SIADH.

**Clinical Pearl / High-Yield Fact**
In SIADH, the key is to identify and treat the underlying cause, while managing hyponatremia with **vasopressin receptor antagonists** or fluid restriction.

**Correct Answer:** D. Tolvaptan