A 12-month-old girl is seen for episodic abdominal pain and vomiting that began earlier in the day. The mother reports temper-tantrum-like behavior followed by crying. The child could be soothed for several minutes before the behavior resumed. The mother said the child had had multiple bowel movements during the day and vomited twice. Past medical history is significant for premature birth (at 37 weeks); incision and drainage for abscess-related MRSA of the jaw. She is otherwise healthy. Vital signs are: temperature, 37.4°C (99.4°F); heart rate, 130 beats/min; respiratory rate, 36 breaths/min; blood pressure, 102/60 mm Hg; SpO2, 99% on room air. The child weighs 7.4 kg. Watch the video for the primary clue to the diagnosis.