A 42-year-old woman with limited cutaneous systemic sclerosis (CREST syndrome) presents with 18 months of progressive exertional dyspnea, palpitations, and two episodes of syncope during exertion. On examination, she has a loud P2, right ventricular heave, elevated JVP with prominent A waves, and hepatojugular reflux. Chest X-ray shows prominent central pulmonary arteries with peripheral oligemia and mild cardiomegaly. ECG demonstrates right axis deviation, RV hypertrophy, and T-wave inversions in V1–V3. Pulmonary function testing is performed, and the pattern marked **B** in the diagram is observed. Which of the following is the most appropriate next diagnostic step to confirm the suspected diagnosis?