An 18-month-old boy presents with a 1-year history of poor weight gain and vomiting. He weighed 3.3 kg when born at term and was growing well until 6 months of age, when he was switched from human milk to formula. His appetite is poor and he vomits intermittently, some weeks with every meal. The emesis is nonbilious, is nonbloody, and usually occurs within 30 minutes of eating but occasionally several hours after eating. The vomiting seems to be worse when he is lying down and has not improved with metoclopramide, lansoprazole, or formula change. He has had no swallowing difficulty or choking. The parents report no recent fever, diarrhea, rash, sick contacts, trauma, or travel. His development is normal.

Physical examination reveals a playful but thin boy. His temperature is 36.7°C, heart rate is 130 beats/min, blood pressure is 92/58 mm Hg, respiratory rate is 26 breaths/min, and oxygen saturation...

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