Which of the following is a cause of hypernatremia?

• A. Water intoxication
• B. Hyponatremia
• C. Excessive free water administration
• D. Fluid deprivation in patients who cannot respond to thirst

Answer: (D)

Hypernatremia happens when there is a free water deficit relative to sodium. If a patient cannot respond to thirst, they won’t drink water even when they need it, so free water is not replaced. Ongoing water losses without adequate replacement create a deficit that concentrates sodium in the extracellular fluid, raising serum sodium levels. In contrast, taking in too much water dilutes sodium and leads to hyponatremia, not hypernatremia; Excessive free water or water overload washes out sodium, while this scenario specifically involves insufficient water intake due to impaired thirst response.