Welcome to Health care magic. Aortic knuckle - is a major vessel arises from left ventricle ( cardiac chamber) and passes upward in direction and some where at the hint of clavicle bone region its again descended and goes through the chest, abdomen and pelvis - with multiple branches and gives oxygenated blood supply. On a normal x-ray its only a border seen at the descending area of aorta. But as a age related changes, hypertensive patients and cardiac pathology this vessel is seen prominent on the left side to the sternum. ( its a finding but not a pathology always) cardia should be evaluated for reason. Good luck. Hope it helps you. Wish you a good health. Anything to ask ? do not hesitate. Thank you.