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': 'The aganglionic segment is encountered in which part of the 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: Congenital aganglionic megacolon (Hirschsprung disease) \n\nHirschsprung disease is\xa0a congenital disorder characterized by aganglionosis of a portion of the intestinal tract.\nAn intestinal segment lacks both Meissner submucosal and Auerbach myenteric plexus. This leads to func\xadtional obstruction and intestinal dilation Proximal to the affected segment.\nHistological findings are:-\n\n\nThe absence of ganglion cells and ganglia in the muscle wall and submucosa of the affected segment.\nThickening and hypertrophy of nerve trunk.\n\n\nThe rectum is always affected with the involvement of more proximal colon to the variable extent most cases involve the rectum and sigmoid only.\nProximal to the aganglionic segment, the colon undergoes progressive dilation and hypertrophy.\nWith time, the proximal innervated colon may become massively distended megacolon.', 'role': 'assistant'}]