This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Imaging of Interstitial Lung Disease, Subject name: Radiology
[{'content': "Westermark's sign is seen in: \nSelect and explain. \nA. Pulmonary embolism, B. Pulmonary sequestration, C. Pulmonary alveolar proteinosis, D. A.B.P.A (Allergic Broncho-pulmonary Aspergillosis)", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. (a) Pulmonary embolismRef: Harrison 19th ed. /1633Westermark sign is due to focus of oligemia in pulmonary embolism (PE). The sign develops due to following reasons:* Dilation of the pulmonary arteries proximal to the embolus* Collapse of the distal vasculature creating the appearance of a sharp cut off on chest X rayThe chest x-ray is normal in the majority of PE cases, the Westermark sign is seen in 2% of patients', 'role': 'assistant'}]