A previously healthy 2-month-old boy is brought to the emergency department with 3 days of decreased oral intake and 5 days of constipation. Before this illness, he had been drinking 28 to 32 oz of formula and having 1 to 2 soft stools daily. His mother reports that when the baby is offered a bottle he cries and does not want to eat. He has had constipation for which she gave him a glycerin suppository, resulting in passage of stool. Review of systems is negative for fever, congestion, cough, vomiting, or diarrhea. He has had no sick contacts. His medical history is significant for his being an appropriate-for–gestational age infant born via cesarean delivery at 38 4/7 weeks. He passed stool in the first 24 hours of life. Family history is noncontributory.

On initial examination his weight is 5.7 kg, length is 62 cm, and head circumference is 42 cm....

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