<s>[INST] context: Phrenic nerve lies in the superior mediastinum and not in the posterior mediastinum. All other choices lies within the posterior mediastinum.Posterior mediastinum: The region containing anatomic structures deep to the pericardial sac, including the thoracic poion of the descending aoa, the azygos system of veins, the thoracic duct, the esophagus, and the vagus and sympathetic nerves. Also Know:Superior mediastinum: The region superior to the sternal angle containing the aoic arch and its three branches, the superior vena cava (SVC) and the brachiocephalic veins, the trachea, the esophagus, and the phrenic and vagus nerves. The superior mediastinum also contains the thymus; however, in an adult, the thymus is usually atrophied and presents as a fatty mass.Anterior mediastinum: The region between the sternal angle, the deep sternal surface, the pericardial sac, and the diaphragm. The anterior mediastinum contains fat and areolar tissue and the inferior pa of the thymus or its remnant.Middle mediastinum: This region contains the pericardial sac and hea. [/INST] question: Which one of the following DOES NOT lie within the posterior mediastinum? | options: A: Phrenic nerve B: Vagus nerve C: Esophagus D: Thoracic aoa | answer: A </s>