A 6-week-old girl was born to a healthy mother via uncomplicated vaginal delivery at full term and subsequently had a normal newborn screen. She had a birthweight of 4.48 kg (large for gestational age, 97% percentile). At her 1-month health supervision visit, she weighed 3.7 kg. Although offered 2 to 3 oz of pumped human milk each feed, she was taking approximately 1 oz every 2 hours without cyanosis, tachypnea, or sweating. She had no trouble latching onto the bottle, a normal suck and swallow, and no choking with feeds. After feeding, she had frequent nonbloody/nonbilious vomiting episodes that most often occurred soon after a feed, but occasionally a few hours later. Her primary care provider recommended fortifying the human milk with formula to 22 kcal/ounce, but it was not well-tolerated due to vomiting. On a repeat weight check 2 weeks before admission, she demonstrated continued poor weight gain and...

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