**Core Concept**
Chronic hyponatremia is a common electrolyte disorder characterized by low sodium levels in the blood, often resulting from excessive water retention, inappropriate antidiuretic hormone (ADH) secretion, or other underlying medical conditions. The cornerstone of therapy aims to correct the underlying cause and gradually increase serum sodium levels to prevent osmotic demyelination syndrome.

**Why the Correct Answer is Right**
The cornerstone of therapy for chronic hyponatremia is the administration of **fludrocortisone** (a synthetic mineralocorticoid) and **fluconazole** (a weak antagonist of ADH). This combination helps to correct the underlying cause by increasing sodium reabsorption in the collecting ducts of the kidneys and blocking ADH-mediated water reabsorption. **Fludrocortisone** acts by increasing the expression of sodium channels in the collecting ducts, while **fluconazole** blocks the action of ADH on V2 receptors, thereby reducing water reabsorption.

**Why Each Wrong Option is Incorrect**
**Option A:** **Hemodialysis** is not a first-line treatment for chronic hyponatremia, as it can rapidly correct sodium levels but also risks inducing osmotic demyelination syndrome.
**Option B:** **Vasopressin receptor antagonists** are effective in treating hyponatremia but are not the cornerstone of therapy due to their high cost and potential side effects.
**Option C:** **Demeclocycline** is a tetracycline antibiotic that can induce nephrogenic diabetes insipidus, which is not the cornerstone of therapy for chronic hyponatremia.

**Clinical Pearl / High-Yield Fact**
When treating chronic hyponatremia, it's essential to correct the underlying cause and gradually increase serum sodium levels to prevent osmotic demyelination syndrome. A general rule of thumb is to increase sodium levels by 1-2 mEq/L per hour, while closely monitoring renal function and electrolyte levels.

**Correct Answer:** C. Demeclocycline