A 6-week-old female infant, born at term via spontaneous vaginal delivery after an uncomplicated pregnancy, is brought into the emergency department (ED) for insufficient weight gain and new-onset diarrhea and vomiting. The infant has a history of poor weight gain from birth. Her birthweight was 3.49 kg at 39 weeks of gestation and she was discharged from the hospital after a 1-day hospital stay. Newborn screening results were normal. She was seen by her pediatrician on day 5 after birth and found to have a 14% weight loss and hyperbilirubinemia, for which she was admitted to an outside hospital for phototherapy for 1 day. The infant exhibited sufficient weight gain and was discharged from the hospital. During the first month of age, the mother reported having poor milk production, for which she supplemented with 20 kcal/oz formula. The mother continued to supplement for 3 weeks until she felt her milk...

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