Final answer:
A definitive imaging study, specifically a computed tomography (CT) scan with contrast of the chest, is the next step for further evaluation of a patient with a widened mediastinum seen on chest X-ray after an automobile crash.
Step-by-step explanation:
A 46-year-old man with a widened mediastinum on a chest X-ray (CXR) following an automobile crash likely needs further evaluation to rule out serious injuries such as an aortic rupture or other vascular injury. The next step in the management of a patient with a widened mediastinum on portable anterior-posterior (AP) CXR should be a more definitive imaging study, typically a computed tomography (CT) scan with contrast of the chest. This can help to better visualize the structures of the mediastinum and assess for any potential injuries or abnormalities like aortic dissection, which is a critical condition that requires prompt diagnosis and treatment.