What is the most critical nursing intervention for a patient with hypernatremia due to dehydration?

Hypernatremia is an elevated serum sodium concentration (>145 mmol/L) typically caused by free water loss exceeding sodium loss, leading to cellular dehydration, neuronal shrinkage, altered mental status, and risk of intracranial hemorrhage if corrected too rapidly due to osmotic fluid shifts. Rationale: A. Frequent vital sign monitoring is important for detecting hemodynamic instability, but it does not directly correct the underlying free water deficit or serum sodium imbalance. It is supportive rather than definitive treatment. B. Hypotonic IV fluids (such as 0.45% sodium chloride or D5W) provide free water to gradually correct hypernatremia by lowering serum sodium concentration and restoring intracellular hydration. Slow administration is critical to prevent cerebral edema from rapid osmotic shifts. C. Increasing oral water intake may help mild cases, but in significant dehydration or symptomatic hypernatremia, oral replacement is often insufficient or unsafe. It is not the most critical intervention in moderate to severe cases. D. Isotonic fluids (0.9% sodium chloride) are used for initial volume resuscitation in hypovolemia but do not correct serum sodium elevation. They may even worsen hypernatremia if used alone without free water replacement.