A 25-year-old woman presents with progressive dyspnea and chest discomfort for one day. She delivered her second child two days ago. Labor and the antenatal period were free of complications. Her first pregnancy was uncomplicated. Her medical and surgical histories are unremarkable. She is not on any medications. A complete blood count is ordered. Her hemoglobin level is 11.8 g/dL (normal:11-18). The other indices are within normal limits.