<prompt> A 75 - year - old lady with severe chronic obstructive pulmonary disease COPD presents with acute breathlessness. The chest X - ray shows there is no focal consolidation. She is afebrile, but D - dimers are markedly elevated. Which of the following is best investigation to diagnose this condition ?MRIVentilation - perfusion VQ scanHigh resolution CT thoraxCT pulmonary angiography  <response>: The patient has a possible Pulmonary embolus. Gold standard for diagnosis of PEa Pulmonary angiography VQ is difficult to interpret in patients with cardiorespiratory disease. VQ is IOC in iodine allergics and pregnancy. 