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': 'All the following are radiological features of mitral stenosis except: \nSelect and explain. \nA. Straight left hea border, B. Oligaemia of the upper lung fields, C. Pulmonary haemosiderosis, D. Lifting of the left bronchus', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. Oligaemia of the upper lung fields', 'role': 'assistant'}]