Which posterior mediastinal structure is most closely applied to the posterior surface of the pericardial sac?

The esophagus is closely related to the posterior surface of the pericardial sac. After coming from the hea, the aoa arches over the left pulmonary aery and left bronchus. Eventually, just above the diaphragm, this vessel is posterior to the esophagus. The azygos vein, on the right side of the thorax, arches over the right pulmonary aery and bronchus. It is also posterior to the esophagus. The thoracic duct is posterior to the esophagus as well and does not contact the pericardial sac. Finally, the trachea is superior to the hea.