Final answer:
The middle adult client with unstable vital signs should receive treatment first in the emergency department triage, as this situation may indicate a life-threatening condition needing immediate attention.
Step-by-step explanation:
A nurse working in an emergency department is triaging four clients and needs to recommend which client should receive treatment first. In triage, priority is given to those who have life-threatening conditions and require immediate attention. Among the clients listed, the priority should be given to the middle adult client who has unstable vital signs. Unstable vital signs can indicate critical conditions such as shock, major cardiac events, severe infection, or other emergencies that can lead to rapid deterioration or death if not immediately addressed.
The other patients, while possibly uncomfortable or in pain, do not present with symptoms suggestive of immediate life-threatening conditions. An older adult with constipation of 4 days, a preschooler with a skin rash, and an adolescent with a closed fracture may require medical attention but do not need to be seen before someone with unstable vital signs.