Which solution is appropriate to dilute intravenous phenytoin for a patient with seizures?

Normal saline solution is the appropriate choice for diluting intravenous phenytoin. This drug is known to be sensitive to pH and can precipitate when mixed with solutions that alter its solubility. Normal saline, which has a pH close to that of the physiological conditions in the body, minimizes the risks associated with precipitation or degradation of the drug.

Other solutions like dextrose 5% in water can introduce glucose into the mixture, which is not ideal since phenytoin may precipitate in such solutions, limiting its effectiveness. Ringer's lactate solution contains calcium, which can lead to further complications when mixed with phenytoin and is not recommended. Hypertonic saline solution has a high concentration of salt, which could affect the overall balance of electrolytes and potentially make the phenytoin less effective or cause unwanted side effects.

Using normal saline not only ensures the solubility of phenytoin but also maintains its pharmacokinetic properties, making it the safest and most effective choice for intravenous administration.