<prompt> Which of the following diagnostic techniques is most specific for pulmonary embolismPulmonary angiographyVentilation lung scanningPerfusion lung scanningAerial blood gas analysis  <response>: CT pulmonary angiography is the first-line diagnostic test for pulmonary thromboembolism.It has the advantages of visualising the distribution and extent of the emboli,or highlighting an alternative diagnosis,such as consolidation,pneumothorax or aoic dissection.A definitive diagnosis of Pulmonary embolism depends on visualisation of an intraluminal filling defect in more than one projection.Secondary signs of PE include abrupt occlusioncut offof vessels,segmental oligemia or avascularity, a prolonged aerial phase with slow filling,and touous,tapering peripheral vessels. Reference:Harrisons Medicine-18th edition,page no:2174;Davidsons Medicine-22nd edition,page no:722. 