**Core Concept**
The Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is characterized by excessive release of antidiuretic hormone (ADH), leading to water retention and resulting in hyponatremia. This condition is often seen in patients with small cell lung cancer, central nervous system disorders, and certain medications.

**Why the Correct Answer is Right**
The treatment of choice for SIADH involves fluid restriction to prevent further water retention and allow the sodium levels to normalize. This approach is based on the principle that reducing water intake will decrease the effective circulating volume, thereby decreasing ADH secretion. Additionally, administering hypertonic saline may be necessary to rapidly correct severe hyponatremia. The goal of treatment is to increase sodium levels gradually to prevent central pontine myelinolysis.

**Why Each Wrong Option is Incorrect**
**Option A:** Demeclocycline is incorrect because it is actually used to treat SIADH by inducing nephrogenic diabetes insipidus, which counteracts the effects of ADH. However, it is not the first-line treatment.
**Option B:** Hydrocortisone is incorrect because while it can correct SIADH in patients with ectopic ACTH-producing tumors, it is not the primary treatment for SIADH.
**Option C:** Vasopressin receptor antagonists (V2 receptor antagonists) are incorrect because they are a newer class of medications that can counteract the effects of ADH, but they are not the first-line treatment for SIADH.

**Clinical Pearl / High-Yield Fact**
Fluid restriction is a crucial component of SIADH treatment, and patients should be instructed to limit their fluid intake to 800-1000 mL/day to prevent further water retention and allow sodium levels to normalize.

**Correct Answer: C. Fluid restriction and hypertonic saline administration.**