In which condition would you expect a widening mediastinum on a chest X-ray?

A widening mediastinum on a chest X-ray is typically indicative of a dissecting aortic aneurysm. This condition involves an abnormal separation within the layers of the aortic wall, leading to the accumulation of blood and resulting in an expansion of the mediastinal space as the dissection progresses.

In a dissecting aortic aneurysm, the comparison of the aorta's normal contour is disrupted, and the presence of a false lumen can create a noticeable widening that is clearly evident on imaging. Furthermore, this condition is often accompanied by severe chest pain that may radiate to the back, and it requires immediate medical intervention due to the risk of rupture or further complications.

While other conditions such as pneumonia and bronchitis can cause mediastinal shifts, they typically do not result in a widening of the mediastinum itself. Therefore, the association of a widening mediastinum is most specifically linked to a dissecting aortic aneurysm.