Case 3: An Infant with Severe Dehydration, Weight Loss, and Abnormal Newborn Screening Test Results

A female infant is born at 37 weeks’ gestation via vaginal delivery to a gravida 4, para 3 woman. The woman’s pregnancy and delivery are uncomplicated. The infant’s birthweight is 2,680 g. She is discharged from the hospital in good condition; at home, she breastfeeds well and gains weight appropriately; on day 10, her weight is 2,778 g (4% above birthweight). On day 12, the mother notes that the infant has begun to breastfeed poorly. On day 14, formula feeding is attempted, but her intake remains poor, with an associated decrease in urine output. On day 16, she presents to an outpatient clinic, where she appears cachectic and severely dehydrated but responsive. Her weight is 1,760 g, a decrease of more than 1 kg (37%) from 6 days ago. She is transferred to the NICU for further evaluation and treatment. The infant’s newborn screening results from day 1 are normal; however, repeat newborn screening performed on day 10 shows an abnormality in the fatty acid oxidation profile, with malonyl carnitine levels of 0.68 μmol/L (normal ≤0.5 μmol/L). The patient’s initial laboratory tests were notable for the following results: sodium, 174 mEq/L (174 mmol/L); potassium, 7.2 mEq/L (7.2 mmol/L); chloride, 141 mEq/L (141 mmol/L); bicarbonate, 7 mEq/L (7 mmol/L); blood urea nitrogen, 273 mg/dL (97 mmol/L); creatinine, 5.61 mg/dL (428 μmol/L); glucose, 1,266 mg/dL (70.2 …