Which sign on a chest x-ray may NOT suggest aortic rupture?

Mediastinal emphysema is not typically indicative of aortic rupture. This sign refers to air being present in the mediastinum, which can occur due to various causes such as trauma to the airways or chest, lung rupture, or rupture of the esophagus. While it can be associated with thoracic injury, its presence does not specifically suggest aortic rupture.

The other signs listed are more directly correlated with potential aortic injury. For instance, a "pleural cap" can indicate bleeding or fluid accumulation, which might accompany an aortic injury. The obliteration of the aortic knob suggests that there may be a significant injury leading to bleeding, as it typically presents in cases of aortic dissection or rupture. Deviation of the trachea to the right can suggest mass effect from a hematoma or other injury affecting the thoracic structures, further implicating a serious vascular injury such as that of the aorta.

In summary, while mediastinal emphysema can occur with various thoracic issues, it is not a definitive sign of aortic rupture, distinguishing it from the other options that more directly indicate such an injury.