You are working at the triage desk in the ED when a young man on crutches approaches the desk. He appears moderately short of breath. He states he had a cast put on his left leg seven days ago after surgery for a ruptured Achilles tendon. This morning, while he was watching television, he suddenly became short of breath. He has a history of asthma, for which he takes Xopenex as needed, and is taking Tylenol with codeine for pain related to his surgery. The patient's lung sounds are clear and equal, SpO2 is 90 percent on room air, heart rate is 100 and regular, respirations are 24 and slightly labored, blood pressure is 128/88 mmHg, and the patient is afebrile. These findings are most consistent with:

A) allergic reaction to codeine.
B) asthma exacerbated by recent anesthesia.
C) pneumonia secondary to recent anesthesia.
D) pulmonary embolism associated with immobilization of the lower extremity.

D