<prompt> Westermark sign is seen in:Pulmonary embolismPulmonary sequestrationPulmonary alveolar proteinosisPneumothorax  <response>: Ans. a Pulmonary embolism Ref- Westermark sign is a sign of pulmonary embolus seen on chest radiographs. It is one of several described signs of pulmonary embolus on chest radiographs. Pathology The theory behind the sign is either obstruction of the pulmonary artery or distal vasoconstriction in hypoxic lung 3. In one study PIOPED this sign was present on 10 of chest x-rays of patients with confirmed pulmonary embolus 2. Radiographic features Plain radiograph focal peripheral hyperlucency secondary to oligaemia resulting in a collapsed appearance of vessels distal to the occlusion 2-4 central pulmonary vessels may also be dilated 3 History and etymology The Swedish radiologist Nils Johan Hugo Westermark 1892-1980 7 first described this sign in 1938 5. Westermark was also an accomplished sailor and won a silver medal, as a member of the Swedish team, at the 1912 Olympics in Stockholm 7. Differential diagnosis emphysema 2 CXR may be normal, or may show Focal oligemia of affected segment Westermark sign Q Dilated right descending pulmonary artery Pallas sign Q Wedge-shaped opacities above the diaphragm Hamptons Hump Q Small pleural effusion 