A 25-year-old graduate student presents to the university health center complaining of acute shortness of breath. The patient states that he returned home yesterday from a non-stop flight from Sydney, Australia. Chest x-ray show multiple infiltrates. EKG is significant for tachycardia. This patient is a high risk for? Which ancillary test would you order to confirm your suspicion? Explain why the patient is experiencing shortness of breath?