Final answer:
The nurse should anticipate administering an antiemetic medication to the 18-month-old toddler with gastroenteritis and dehydration.
Step-by-step explanation:
The nurse should anticipate administering an antiemetic medication to the 18-month-old toddler with gastroenteritis and dehydration. Gastroenteritis causes emesis (vomiting), and the toddler is still experiencing vomiting despite the oral rehydration solution. An antiemetic medication can help reduce or stop the vomiting, allowing the toddler to retain the fluid and rehydrate effectively.