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: Surgery
A 56 year old woman has not passed stools for the last 14 days. Xray shows no air fluid levels. Probable diagnosis is : 
Select and explain. 
A. Paralytic ileus, B. Aganglionosis of the colon, C. Intestinal pseudo-obstruction, D. Duodenal obstruction
C: Intestinal pseudo-obstruction
Answer: C: Intestinal pseudo-obstruction 
Explanation: Air fluid levels are unusual in intestinal pseudoobstruction and should raise the possibility of mechanical obstruction. Ref: Harrison's Internal Medicine, 14th Edition, Page 1650 and 15th Edition, Page 1697; Update Gastroenterology 2004: New Developments in the Management of Benign gastrointestinal Disorders, 2004, Page 103.