A school-age child diagnosed with leukemia experienced severe nausea and vomiting when receiving chemotherapy for the first time. Which is the most appropriate nursing action to prevent or minimize these reactions with subsequent treatments?

A. Administering an antiemetic at least 30 minutes before chemotherapy begins is correct because prevention is more effective than treatment after symptoms appear. Administering antiemetics prophylactically helps reduce the severity and incidence of chemotherapy-induced nausea and vomiting, improving patient comfort and adherence to treatment. B. Encouraging the child to remain NPO until nausea subsides is incorrect because prolonged fasting does not prevent chemotherapy-induced nausea and may contribute to dehydration and nutritional deficits, especially important in children undergoing cancer treatment. C. Encouraging drinking large amounts of fluids is incorrect because forcing large volumes may worsen nausea or vomiting. Fluid intake should be adequate but balanced to prevent discomfort. D. Administering an antiemetic after the child has nausea is incorrect because reactive treatment is less effective than prophylactic administration. Once severe nausea and vomiting have occurred, they are harder to control and can impact hydration, nutrition, and overall treatment tolerance.