**Core Concept**
Hypernatremia is a condition characterized by an elevated serum sodium level (usually >145 mmol/L) that can lead to dehydration, seizures, and even death if left untreated. The management of hypernatremia involves correcting the underlying cause, administering free water to dilute the sodium concentration, and using medications to enhance free water reabsorption.

**Why the Correct Answer is Right**
In managing hypernatremia, the primary goal is to correct the serum sodium level while avoiding rapid changes that can lead to cerebral edema. Free water administration is essential, and options like **Demeclocycline (A)**, **Vasopressin (B)**, and **Hypertonic saline (C)** are used in specific scenarios. Demeclocycline can be used in cases of persistent hyponatremia by inducing nephrogenic diabetes insipidus, reducing free water reabsorption. Vasopressin can be used to treat central diabetes insipidus, which can be a cause of hypernatremia. Hypertonic saline is used to rapidly correct severe hyponatremia or in cases of acute water intoxication.

**Why Each Wrong Option is Incorrect**
**Option A:** Demeclocycline is used to treat hypernatremia by inducing nephrogenic diabetes insipidus, making it incorrect for this scenario.
**Option B:** Vasopressin can be used in cases of central diabetes insipidus to reduce free water loss, making it incorrect for this scenario.
**Option C:** Hypertonic saline is used to rapidly correct severe hyponatremia, making it incorrect for this scenario.

**Clinical Pearl / High-Yield Fact**
The "osmole gap" can be used to estimate the severity of hypernatremia and guide treatment. A high osmole gap suggests the presence of an osmotically active substance, while a low osmole gap suggests free water excess.

**Correct Answer:** A. Demeclocycline is used to treat hypernatremia by inducing nephrogenic diabetes insipidus, making it incorrect for this scenario.