This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory System, Subject name: Radiology
[{'content': 'A lesion with sharp outlines extending above the clavicles suggests? \nSelect and explain. \nA. Anterior mediastinal lesion, B. Posterior mediastinal lesion, C. Cardiac lesion, D. All of the above', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans BThe anterior mediastinum ends below the clavicle while the apical lobes of upper lung lobes extend superior to the clavicles more posteriorly hence if a mass lies in the posterior mediastinum its margins would be lined by air filled lungs and hence seen as sharp margins. This is an extension of the Silhouette sign - the Cervico-thoracic sign.', 'role': 'assistant'}]