A 9-week-old girl with a normal perinatal course, who has been otherwise well, presents to the pediatric emergency department (ED) with a 2-week history of fussiness when lying supine. She has 3 days of decreased oral intake and loss of her social smile. She is brought to her pediatrician for inconsolability and new-onset diarrhea. A complete blood cell (CBC) count was performed revealing leukocytosis (white blood cell count, 40,000/uL (40 × 109/L), prompting referral to the ED.

In the ED she develops a fever (102°F [38.9°C]). The remainder of her vital signs are as follows: heart rate, 170 beats/min; respiratory rate, 38 breaths/min; blood pressure, 98/62 mm Hg; and oxygen saturation 98% on room air. Her skin is warm and flushed, and she is crying inconsolably, with irritability consistently worsened by abdominal palpation, but no fullness or masses are appreciated. The perianal examination reveals slow leakage of thin,...

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