This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Medicine
[{'content': 'A 55 year old man who has been on bed rest for the past 10 days, complains of breathlessness and chest pain. The chest X-ray is normal. The next investigation should be \nSelect and explain. \nA. Lung ventilation-perfusion scan, B. Pulmonary aeriography, C. Pulmonary venous angiography, D. Echocardiography', 'role': 'user'}, {'content': "Answer: A \nExplanation: Lung scanning has become a second-line diagnostic test for PE, used mostly for patients who cannot tolerate intravenous contrast. Small paiculate aggregates of albumin labeled with a gamma-emitting radionuclide are injected intravenously and are trapped in the pulmonary capillary bed. The perfusion scan defect indicates absent or decreased blood flow, possibly due to Pulmonary EmbolismRef: Harrison's 18/e p2173, 17/e p1652, 1663", 'role': 'assistant'}]