A 30 year old man with no significant medical history presents to the emergency department with acute pleuritic chest pain and dyspnea. He recently had knee arthroscopy and has been largely immobile for a week. Heart rate is 112 per min, blood pressure is 124 over 78 mm Hg, respiratory rate is 24 per min, and oxygen saturation is 93 percent on room air. Physical examination reveals mild unilateral leg swelling. His Wells score suggests intermediate probability for pulmonary embolism. What is the most appropriate initial diagnostic test?