Q. A 3-month-old male infant is brought to your clinic by his parents for evaluation of chronic constipation and failure to thrive. The child had a normal vaginal delivery with no complications at term. There was reportedly no passage of meconium within the first 48 hours of life. He has not gained any weight since birth, and only has one bowel movement per week. The baby's birth weight was 3.5 kg (7.7 lb) and he was in the 50th percentile for weight and length. On physical examination, the infant's weight is below the 3rd percentile today. His cardiopulmonary assessment is within normal limits. His abdomen is distended and diffusely tender. No organomegaly is appreciated. You order a radiologic evaluation, including an abdominal X-ray and a barium enema (see image). The X-ray shows a distended colon with multiple air-fluid levels, and the barium enema shows a prominent transition zone. Based on the information you have at this point in time, which of the following is the most likely underlying etiology for this patient’s diagnosis?