A nurse is caring for a client who has hyponatremia and is receiving an infusion of a prescribed hypertonic solution. Which of the following findings should indicate to the nurse that the treatment is effective?

The correct answer is D: Improved cognition. In hyponatremia, the brain swells due to low sodium levels, causing neurological symptoms. Hypertonic solutions help raise sodium levels, reducing brain swelling, and improving cognition. Absent Chvostek's sign (A) is associated with hypocalcemia, not hyponatremia. Decreased vomiting (B) is a general symptom improvement but does not directly reflect the effectiveness of treating hyponatremia. Absence of cardiac arrhythmias (C) is important, but it does not specifically indicate improvement in hyponatremia. Improved cognition (D) directly correlates with resolving the underlying issue of brain swelling due to low sodium levels, making it the most appropriate indicator of treatment effectiveness.