A 29-year-old lady referred to you for assessment of SOB. She is an active person who ran 2 marathons in the past. Currently, she noticed SOB after running 3 km, which is unusual for her. This has been going on for the past 2 years. when she started to notice a gradual decrease in her exercise capacity. No CP, cough, LL edema, or SOB during bedtime. She has no significant past medical history, and she is not on any medication. No prior history of smoking or exposure to environmental or occupational dusts or gases. Physical examination revealed a normal body mass index of 23 kg/ m2. Normal vital signs, and on examination, there was normal S1 and loud S2; otherwise, it was normal. CXR was normal. CT chest showed normal lung parenchyma with a mildly dilated main pulmonary artery. PFT showed normal spirometry with a lower limit of normal DLCO. The echocardiogram showed high systolic pulmonary artery pressure The next step in the investigation is: