A 66-year-old woman comes to the emergency department with sudden onset dyspnea for 3 hours. There is no fever, cough, chest pain, nausea, vomiting, faintishness, or calf pain. This is her first such episode. She had an uncomplicated hemicolectomy for colorectal carcinoma last week. She is on acetaminophen (paracetamol) for analgesia. She is not on any other medications or supplements. She has no other comorbidities. She does not smoke. Her oxygen saturation is 94% on room air.