A newborn with Down syndrome has constipation. Rectal examination shows no stool in the rectal ampulla. The proximal rectosigmoid is dilated on radiograph. The mechanism most likely responsible for the patient's disease is
An imperforate anus An inflammatory lesion in the rectosigmoid area Inspissated meconium owing to a defect in exocrine secretions Absent ganglion cells in the submucosal and myenteric plexus