This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Small & Large Intestine, Subject name: Surgery
How to differentiate mechanical obstruction from paralytic ileus? 
Select and explain. 
A. Elevation of hemi diaphragm, B. Absence of rectal shadow, C. Multiple air fluid levels, D. Abdominal distension
B: Absence of rectal shadow
Answer: B: Absence of rectal shadow 
Explanation: Ans. (b) Absence of rectal shadowRef: Bailey and Love 26th edition, Page 1190* The important point to differentiate paralytic ileus and mechanical obstruction in X-ray is there will be absence of rectal gas shadow in mechanical obstruction.* Clinically bowel sounds will be absent in paralytic ileus with no pain.