This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pediatrics
Aganglionic segment is encountered in which pa of colon in case of Hirschsprung's disease ? 
Select and explain. 
A. Distal to dilated segment, B. In whole colon, C. Proximal to dilated segment, D. In the dilated segment
A: Distal to dilated segment
Answer: A: Distal to dilated segment 
Explanation: Ans. is 'a' i.e., Distal to dilated segmentCongenital aganglionic megacolon (Hirschsprung disease) Hirschsprung disease is a congenital disorder characterized by aganglionosis of a poion of the intestinal tract.An intestinal segment lacks both Meissner submucosal and Auerbach myenteric plexuses. This loss of enteric neural coordination leads to functional obstruction and intestinal dilation proximal to the affected segment.Histological findings are ?i) Absence of ganglion cells and ganglia in the muscle wall and submucosa of the affected segment. Thickening and hyperophy of nerve trunk.ii) Rectum is always affected with involvement of more proximal colon to variable extent --> most cases involve the rectum and sigmoid only.o Proximal to the aganglionic segment, the colon undergoes progressive dilation and hyperophy. o With time, the proximal innervated colon may become massively distended --> megacolon.