Answer is A (Lung perfusion -- ventilation scan): The clinical picture depicted in the question is suggestive of a case of pulmonary thromboembolism. As the patient has a high clinical likelihood, the first investigation should be a non invasive imaging modality in accordance with the diagnostic strategy for PTE outlined in Harrison's textbook. CT of the chest with intravenous contrast (Multidetector CT) is the imaging test of choice but it has not been provided amongst the options. As CT scan of the chest is not provided amongst the options, a Lung Ventilation Perfusion Scan is the modality of choice. Diagnostic Strategy in cases of suspected Pulmonary Thromboembolism An elderly man on bed rest for past 10 days: predisposition for deep vein thrombosis. (immobilization) Complaints of breathlessness and chest pain: indicative of pulmonary embolisation. (A chest X-Ray in patient with PTE may well be normal or near normal - Harrison's 15th/1510) Remember: Most specific test for establishing a diagnosis of pulmonary thromboembolism is selective pulmonary angiographyQ