<prompt> Most reliable sign of injury to intrathoracic aoa isPresence of apical cap of pleural fluidDepression of left main stem bronchousObliteration of aoic knob contourFunny looking mediastenum  <response>: Answer- C. Obliteration of aoic knob contourThe most reliable of these signs for the blunt aoic iniury is loss ofthe aoic knob contour. 