A 68-year-old man with no previous medical problems presents to the clinic with shortness of breath and swelling of his ankles. Symptoms have progressed over the past few months. The patient has a regular heart rate and rhythm, an S4 is heard on cardiac auscultation, and inspiratory rales are heard throughout the lower half of the lung fields. Also, 2&#43; pitting edema is evident over the tibial surfaces. Echocardiography reveals an ejection fraction of 60%. Which of the following is most likely to improve this patient’s condition?