A 38-year-old woman is evaluated for exertional dyspnea and fatigue. Her symptoms began 4 months ago and have been steadily worsening. On two recent occasions, she experienced near-syncope walking up two flights of stairs. She notes no shortness of breath at rest or other respiratory symptoms. Medical history is significant for limited cutaneous systemic sclerosis. Her only medication is lansoprazole.
On physical examination, temperature is 36.8 °C (98.2 °F), blood pressure is 122/72 mm Hg, pulse rate is 88/min, and respiration rate is 16/min; BMI is 26. Oxygen saturation breathing ambient air is 94%. Pulmonary examination reveals clear lung fields. Cardiac examination discloses a prominent S2. Sclerotic changes are noted in the bilateral hands and forearms.
Chest radiograph is normal. Pulmonary function testing shows normal spirometry findings and a DLCO of 48% of predicted. CT angiography shows no evidence of pulmonary embolism. Transthoracic echocardiography shows normal left ventricular function, an enlarged right ventricle, and an estimated pulmonary artery pressure of 52 mm Hg.
Which of the following is the most appropriate diagnostic test to perform next?