This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Radiology Q Bank, Subject name: Radiology
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 ? 
Select and explain. 
A. MRI, B. Ventilation - perfusion (V/Q) scan, C. High resolution CT thorax, D. CT pulmonary angiography
D: CT pulmonary angiography
Answer: D: CT pulmonary angiography 
Explanation: The patient has a possible Pulmonary embolus. Gold standard for diagnosis of PEa Pulmonary angiography V/Q is difficult to interpret in patients with cardiorespiratory disease. V/Q is IOC in iodine allergics and pregnancy.