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': 'Most reliable sign of injury to intrathoracic aoa is \nSelect and explain. \nA. Presence of apical cap of pleural fluid, B. Depression of left main stem bronchous, C. Obliteration of aoic knob contour, D. Funny looking mediastenum', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Answer- C. Obliteration of aoic knob contourThe most reliable of these signs for the blunt aoic iniury is loss ofthe aoic knob contour.', 'role': 'assistant'}]