Approach to growth faltering Quiz: Ace Your Exams

An 8-month-old boy presents to the office with four weeks of diarrhea. The parent reports that the stool is foul-smelling, bulky, and greasy. The Infant was born full term without complications and passed meconium at three days of life. Newborn screening results are unknown. There is no history of vomiting, choking, or increased work of breathing with feeds. History is significant for a diagnosis of pneumonia requiring hospital admission at four months of age. Family history is noncontributory. Temperature is 37°C (98.6°F), pulse is 115/min, respirations are 30/min, blood pressure is 85/45 mm Hg, and oxygen saturation is 97% on room air. Weight has dropped from the 55^th percentile to the 7^th percentile. Length has dropped from the 30^th percentile to the 3^rd percentile. Physical examination reveals a pale thin-appearing infant without dysmorphic features or characteristic odors. Pulmonary and abdominal exams are normal. A 2/6 flow murmur is appreciated best at the left lower sternal border. Which of the following can be used to confirm the underlying cause of growth faltering?