A 58-year-old man comes to the physician because of a 3-month history of shortness of breath. He has had orthopnea, paroxysmal nocturnal dyspnea and peripheral edema. He has not had chest pain. He has no history of serious illness and takes no medications. He drinks ten alcoholic beverages daily. His pulse is 100/min and regular, respirations are 20/min, and blood pressure is 130/75 mm Hg. There are jugular venous pulsations 5 cm above the sternal angle. Crackles are heard at both lung bases. Cardiac examination shows an S3. There is 2+ pretibial edema bilaterally. The most appropriate next step in management is administration of which of the following?