Westermark's sign is seen in:

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