A 14-yeаr-оld presents tо the emergency depаrtment with а 12-hоur history of abdominal pain, nausea, and one episode of vomiting. The nurse notes that the pain initially began around the umbilicus but is now localized to the right lower quadrant. The child has a temperature of 100.8°F (38.2°C) and tenderness over McBurney's point. Which nursing action is the priority?