<prompt> A lesion with sharp outlines extending above the clavicles suggests?Anterior mediastinal lesionPosterior mediastinal lesionCardiac lesionAll of the above  <response>: 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. 