Failure to Thrive, Metabolic Acidosis, and Diarrhea in a 7-Week-Old Infant.

A 7-week-old infant presented to the emergency department with fussiness, decreased oral intake, loose stool, and respiratory distress for 2 days. The patient was born full-term with an uncomplicated birth history but had a history of slow weight gain. He was alert, but toxic-appearing at presentation, hypothermic with signs of dehydration, and with respiratory failure. He was found to have severe anion gap metabolic acidosis, hypokalemia, elevated lactate, and hyperammonemia. He responded well to initial resuscitation and was admitted to the ICU for intravenous electrolyte replacement, bowel rest, and respiratory support. A workup was pursued for failure to thrive with severe malnutrition, hyperammonemia, hyperlactatemia, anemia, vitamin D deficiency, and electrolyte abnormalities. After stabilization, he was restarted on enteral feeds and had a recurrence of loose stool and severe electrolyte abnormalities, which were refractory to enteral supplementations and required readmission to the ICU. His hospital course extended several weeks, included several subspecialty consultations, and ended with a surprising diagnosis of exclusion based on his clinical response to therapy.