This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Thorax, Subject name: Anatomy
[{'content': 'The posterior mediastinum contains all of the following Except \nSelect and explain. \nA. The hemiazygos vein, B. The azygos vein, C. The thoracic duct, D. The brachiocephalic veins', 'role': 'user'}, {'content': 'Answer: D \nExplanation: The brachiocephalic veins are in the superior mediastinum. They converge to form the superior vena cava. The separation between the superior mediastinum and the posterior mediastinum is an imaginary horizontal plane at the level of the fouh thoracic veebra. The superior mediastinum contains both brachiocephalic veins and the superior vena cava, the aoic arch and the brachiocephalic, left common carotid and left sub clan aeries, the trachea, esophagus, and pa of the thymus. The posterior mediastinum is the region of the thorax below the plane at the fouh thoracic veebra, between the pleural cavities and behind the pericardial sac.', 'role': 'assistant'}]