Hirschsprung disease is caused by a lack of ganglia in the rectal wall which prevents peristalsis and leads to abdominal distention and failure to pass meconium. It is definitively diagnosed by a rectal biopsy that shows this lack of ganglion cells. An anteroposterior abdominal x-ray was likely performed upon initial concern for failure to pass meconium and abdominal distention and is helpful for ruling out other conditions but is not diagnostic for Hirschsprung disease. An upper gastrointestinal endoscopy is used to detect disorders of the esophagus, stomach, and small intestines. A lower gastrointestinal endoscopy can help rule out other conditions but does not definitively diagnose Hirschsprung disease.