A term infant has not passed meconium for 48 hours. He presents with distension of abdomen and emesis, since one day. Next most appropriate investigation would be

A term infant presenting with failure to pass meconium within 48 hours, abdominal distension, and emesis strongly suggests meconium ileus or Hirschsprung disease. A lower bowel contrast enema is the most appropriate next diagnostic step as it can both diagnose and potentially treat meconium obstruction or delineate transition zones.