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
Cause of posterior mediastinal opacity on PA and lateral view of chest x- ray 
Select and explain. 
A. Bochdalek's hernia, B. Lymph node mass, C. Tortuous innominate artery, D. Aneurysms
A: Bochdalek's hernia
Answer: A: Bochdalek's hernia 
Explanation: Anterior mediastinum - Thymona
Muddle mediastinum - Tortuous innominte artery
Posterior mediastinum - Bochadalek's hernia