A 41-year-old male is admitted to the emergency department with complaints of shortness of breath, dizziness, and sharp chest pain. The large arrow in his chest radiograph indicates the region of pathology. What is this structure?

(d) Source: GAS 180-185; GA 85-88The normal position of the heart as seen in a plain radiograph has the right border of the heart formed by the superior vena cava, right atrium, and inferior vena cava. The left border is formed by the aortic arch superiorly, left pulmonary artery, left auricle, left ventricle, and the apex of the heart inferolaterally. The area indicated by the arrow is just inferior to the clavicle (on the left side), and this marks the location of the arch of the aorta. The superior vena cava and right ventricle would make up the right border. The pulmonary artery and left ventricle would be more inferior.