**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. In SIADH, the body retains too much water, diluting sodium levels in the blood.

**Why the Correct Answer is Right**
In SIADH, the goal is to correct the water retention and hyponatremia by increasing urine output and sodium levels. **Demeclocycline** is a tetracycline antibiotic that has a unique side effect of inducing nephrogenic diabetes insipidus, making it effective in treating SIADH. It works by inhibiting the V2 receptors in the kidneys, which normally respond to ADH to increase water reabsorption. By blocking these receptors, demeclocycline reduces water reabsorption, increasing urine output and correcting hyponatremia.

**Why Each Wrong Option is Incorrect**
* **Option A:** Vasopressin (ADH) is the actual cause of SIADH; administering more of it would worsen the condition.
* **Option B:** Loop diuretics like furosemide increase urine output but do not specifically target the underlying issue of excessive ADH; they can also lead to hypokalemia and other electrolyte imbalances.
* **Option D:** Hydrochlorothiazide (HCTZ) is a thiazide diuretic that can worsen hyponatremia in SIADH by increasing sodium reabsorption in the kidneys.

**Clinical Pearl / High-Yield Fact**
Demeclocycline is a useful treatment option for SIADH, but it's essential to monitor for potential side effects, including nephrogenic diabetes insipidus, photosensitivity, and superinfection.

**Correct Answer:** C. Demeclocycline