A 52-year-old woman presents with progressive exertional dyspnea and chest discomfort. Echocardiogram reveals mean pulmonary artery pressure 30 mm Hg at rest, pulmonary capillary wedge pressure 10 mm Hg, and no significant left heart disease. CT angiography shows no chronic thromboembolic disease. Right heart catheterisation confirms pulmonary arterial hypertension. According to current guidelines, initial therapy for low-risk WHO functional class II pulmonary arterial hypertension includes which of the following?