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': "Cause of posterior mediastinal opacity on PA and Lateral view of chest x- ray? \nSelect and explain. \nA. Bochdalek's hernia, B. Lymph node mass, C. Tortuous innominate artery, D. Aneurysms", 'role': 'user'}, {'content': "Answer: A \nExplanation: Anterior mediastinum - Thymona.\nMiddle\xa0mediastinum\xa0 - Tortuous innominate artery.\nPosterior\xa0mediastinum : Bochdalek's hernia.", 'role': 'assistant'}]