A 65-year-old m an developed sudden dyspnea and chest pain2 days after surgery to remove a gastric carcinoma. Physical examination revealed an anxious m an in severe respiratory distress  with the following vital signs: temperature 99.5°F (37.5°C), pulse 120 bpm , blood pressure 90/50 m m Hg, respirations 28 breaths/m in. A computed tomography scan showed complete obstruction of a branch of the left pulmonary artery. Which of the following drugs should be included in the acute parenteral treatment of this patient?