<prompt> On barium swallow which of the following will cause posterior impression -Left atriumAortic knucklePulmonary slingAberrant right subclavian artery  <response>: Aberrant right subclavian artery arises from aortic arch just distal to the left subclavian artery. It travels behind the esophagus and after crossing the esophagus from left to right it courses as the right subclavian artery. Aberrant right subclavian artery produces the classical appearance of a posterior impression on the proximal esophagus and left sided aortic arch impression as it arises from left side of aortic arch on barium swallow.Plain Radiograph with barium swallowAberrant right subclavian artery :- i Left sided aortic arch impression ii Posterior impression on esophagusAberrant left subclavian artery :- i Right sided aortic arch impressionii Posterior impression on esophagusiii Anterior displacement of esophagusiv Compression of trachea 