A 4-WEEK-OLD male infant was admitted to the hospital for evaluation of failure to thrive. The child was born at 36 weeks' gestation; his birth weight was 2.88 kg. The antenatal course was uncomplicated, but the stomach was not visualized on routine prenatal ultrasound. The child was described as a "poor feeder" who slept a lot and had to be awakened for feedings. Breastfeeding was attempted, but he nursed poorly and was given 20-calorie/oz cow's milk base formula, followed by several formula changes for "constipation" and "diarrhea." The infant would eat 14 to 16 oz of formula per day and required 30 minutes to ingest 1 oz. He had postprandial emesis with almost every feeding, and the emesis was preceded by back arching. No respiratory distress, cough, or color change was noted with feedings.
Physical examination demonstrated an alert male infant with 2.59-kg weight (<5th percentile), 48-cm height (<5th percentile), and