A 2-month-old previously healthy, full-term boy is admitted for intermittently bloody stools. This exclusively breastfed infant has had 2 to 3 blood-streaked diapers per day for the past 4 weeks. He is also noted to be fussier by the parents. There is no history of any recent illnesses, fevers, vomiting, or other signs of bleeding. His oral intake and urine output are normal. His medical history is notable for poorly controlled eczema. He does not use any medications or have any known allergies. There is no family history of any medical problems, including bleeding disorders, gastrointestinal disease, or atopy.

On physical examination, he is noted to be small for age (9th percentile, previously 89th percentile) and has moderate eczema on his face and extremities. The remainder of his examination findings are normal.

Initial laboratory values are significant for normocytic anemia, thrombocytopenia, and eosinophilia: white blood cell count, 16 × 10...

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