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: Pathology
[{'content': 'Aganglionic segment is encountered in which pa of colon in case of Hirschsprung disease ? \nSelect and explain. \nA. Distal to dilated segment, B. In Whole colon, C. Proximal to dilated segment, D. In the dilated segment', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e., Distal to dilated segment Congenital aganglionic megcolon (Hirschsprung disease) o Hirshsprung disease a congenital disorder characterized by aganglionosis of a poion of the intestinal tract. o An intestinal segment lacks both Meissner submucosal and Auerbach myenteric plexuses. This leads to functional obstruction and intestinal dilation Proximal to the affected segment. o Histological findings are :- (i) Absence of ganglion cells and ganglia in the muscle wall and submucosa of the affected segment. (ii) Thickening and hyperophy of nerve trunk. o 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.", 'role': 'assistant'}]