A 13-year-old Hispanic boy has experienced fatigue and diffuse abdominal pain for 2 days. He has had many black, loose stools and today passed bright red blood in his stool. The patient has not had similar complaints in the past and denies fever, vomiting, or recent travel. He has not eaten any unusual or uncooked food and is not taking any medications. No one in the family has similar symptoms. Earlier today, at another hospital, he was found to have anemia, with an Hgb level of 4.2 g/dL (42 g/L). After intravenous access was achieved and provisions made for a safe transfer, he was transported to the tertiary center for further management.

On examination, the boy’s temperature is 99.1°F (37.3°C), heart rate is 114 beats/min, respiratory rate is 26 breaths/min, and blood pressure is 104/72 mm Hg. He looks pale and tired and is tender in the right lower abdominal...

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