A 5-day-old infant is brought by his parents who have noticed progressive abdominal distention and constipation. He has been breastfeeding well, but recently his oral intake is diminished and his abdomen has become distended. Birth history is unremarkable except for delayed passage of meconium (>48 h). Abdominal radiography reveals marked dilation of small bowel and colon. What is the best next diagnostic study?