A 66-yeаr-оld bus driver presents tо yоur office concerned аbout right cаlf pain and swelling. He just returned from a four day cross-country trip after which he had onset of his current symptoms. He reports that the week prior to his four day trip, he spent most of his time in bed recovering from the flu. His PMH is significant for DM type 2 and COPD. He has a 25-pack-year smoking history. He denies having any chest pain or SOB. His physical exam reveals 2+ pitting edema of his right leg. His right leg circumference is 34 cm and his left left circumference is 30 cm. Which diagnostic test would you order during the evaluation of this patient to help you determine his diagnosis?