**Core Concept**
The Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH) is characterized by excessive secretion of antidiuretic hormone (ADH) leading to water retention and hyponatremia. It is crucial to manage SIADH with medications that help correct the water imbalance and normalize electrolyte levels.

**Why the Correct Answer is Right**
The drug of choice for SIADH is tolvaptan, a selective vasopressin receptor antagonist. Tolvaptan blocks the action of ADH on the collecting ducts of the kidneys, thereby reducing water reabsorption and increasing free water excretion. This action helps to correct hyponatremia and improve symptoms in patients with SIADH. Tolvaptan's mechanism of action is particularly useful in SIADH as it does not affect the release of ADH, making it an effective treatment option.

**Why Each Wrong Option is Incorrect**
**Option A:** Demeclocycline is incorrect because it is a tetracycline antibiotic that can induce nephrogenic diabetes insipidus by reducing the responsiveness of the kidneys to ADH. While it can help manage SIADH, it is not the first-line treatment.
**Option B:** Hydrochlorothiazide is incorrect because it is a diuretic that can worsen hyponatremia by increasing sodium loss in the urine.
**Option C:** Furosemide is incorrect because it is a loop diuretic that can also worsen hyponatremia due to its effect on sodium loss in the urine.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that SIADH is a diagnosis of exclusion, and other causes of hyponatremia, such as heart failure, liver disease, and adrenal insufficiency, must be ruled out before diagnosing SIADH. Additionally, tolvaptan should be used cautiously in patients with liver disease, as it can cause liver function abnormalities.

**Correct Answer: D. Tolvaptan**