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
[{'content': 'How to differentiate obstruction from paralytic lens? \nSelect and explain. \nA. Absence of rectal gas shadow, B. Abdominal distension, C. Elevation of hemidiaphragm, D. Multiple air fluid levels', 'role': 'user'}, {'content': 'Answer: A \nExplanation: In x-ray of mechanical obstruction, there will be absence of rectal gas shadow.', 'role': 'assistant'}]