This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
[{'content': "Which among the following are a cause of posterior mediastinal opacity on Posterior-Anterior (PA) and lateral view of chest radiograph? \nSelect and explain. \nA. Tortuous innominate artery, B. Bochdalek's Hernia, C. Enlarged pulmonary artery, D. Thymoma", 'role': 'user'}, {'content': 'Answer: B \nExplanation: Amongst the given options Bochdalek hernia is the posterior mediastinal mass.\nTortuous innominate artery and enlarged pulmonary artery (e.g. aneurysm) are middle mediastinal mass, and thymoma\xa0is anterior mediastinal mass.', 'role': 'assistant'}]