You are evaluating a 2-month-old boy who has a 6-day history of increasing irritability, high-pitched cry, and nonbloody, nonbilious emesis. A term baby, his mother noted that he was fussier and spit up more than her other three children, despite the use of milk-based, soy-based, and elemental formulas. Three weeks ago, they changed to whole cow milk, alternating daily with 2% cow milk. Over the past week, the baby has become increasingly irritable and has been vomiting all feedings for 2 days. He has lost 0.5 kg since birth. He is having regular bowel movements, with no blood in his stool. There has been no fever or contacts with sick people.

On physical examination, the baby is alert, responsive, and in no acute distress. His vital signs are within normal limits for age. His abdomen is not tender, but is distended. No organomegaly is noted, but a large mass is...

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