You are consulted to see a 2-day old baby boy for failure to pass the first meconium stool and an episode of bilious emesis. He was 37-weeks gestation at the time of a normal spontaneous vaginal delivery without any complications. Physical examination reveals a distended abdomen and digital rectal exam results in an explosive expulsion of stool and gas. A contrast enema was ordered and is attached. What is the specific cause (not the diagnosis) of this infant’s condition and what is the next diagnostic step?