Which chest X-ray feature is indicative of aortic dissection?

A widened mediastinal knob on a chest X-ray is indicative of aortic dissection because it reflects the presence of an abnormal amount of blood in the mediastinal space resulting from the dissection itself. In aortic dissection, the layers of the aortic wall can separate, leading to blood tracking between these layers. This can cause the aorta to appear wider than usual on imaging studies, creating an abnormal contour, particularly at the aortic knob, which is the prominence of the aorta as it curves.

In the context of diagnosing aortic dissection, the identification of a widened mediastinal knob suggests significant pathology, such as an expanding hematoma or disrupted aortic architecture, both of which are critical findings in patients suspected of having this life-threatening condition.