A young couple brings in their 18-month-old daughter who has been ill for the past 36 hours. They recount symptoms of fevers as high as 101.9°, vomiting, one episode of diarrhea, and abdominal pain. Other than intermittent bouts of fast breathing and “grunting” noises, she has not had cough or other cold symptoms. An older sibling recently had an upper respiratory infection, with no gastrointestinal symptoms. The child is otherwise in good health with up-to-date immunizations. She currently has a low-grade temperature of 100.4°, with otherwise age-appropriate vital signs. She is not tachypneic. You perform an examination on the tired but alert child and note a slightly distended abdomen and diffuse tenderness with guarding. The chest examination is clear to auscultation. You wonder how to further evaluate this child. Then you grab the order sheet and explain to the parents the tests you are ordering.