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': 'If the right cardiac silhouette is obliterated, it means the pathology involves: \nSelect and explain. \nA. Right middle lobe, B. Right lower lobe, C. Right atrium of hea, D. Right ventricle of hea', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Right middle lobe opacity illustrates the "silhouette sign" on the right hea border. The silhouette sign is is commonly associated with alveolar disease. It consists of obliteration of an anatomic border due to direct contact with an alveolar density. It is also associated with pulmonary collapse, masses and pectus excavatum. Ref: Kritek P., Reilly, J.J. (2012). Chapter e34. Atlas of Chest Imaging. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison\'s Principles of Internal Medicine, 18e.', 'role': 'assistant'}]