What does Hirschsprung's disease involve? SELECT ONE.

In Hirschsprung's disease, there is an absence of myenteric plexus in the upper anal segment (i.e., the most distal portion of the cloaca). In 15%, the myenteric plexus involves only the upper anus; in 70%, the rectum is also involved; and in 15%, part of the colon is also involved. The abnormal segment is contracted; whereas, the dilated bowel is proximal to the diseased segment.