Final answer:
The priority intervention for a nurse caring for a child with symptoms indicative of chickenpox is to isolate the child to prevent the spread of the infection.
Step-by-step explanation:
If a nurse in an emergency department (ED) is caring for a child with a fever and fluid-filled vesicles on the trunk and extremities, the priority intervention should be B. Isolating the child. This presentation is highly indicative of chickenpox, which is a highly contagious viral infection. Isolation is a key step in preventing the spread of infection to others in the ED setting. While other interventions such as administering acetaminophen for fever, applying cold compresses for comfort, and administering antiviral medication are important, they do not address the immediate public health concern of contagion.