A 35-year-old female complains of slowly progressive dyspnea. Her history is otherwise unremarkable, and there is no cough, sputum production, pleuritic chest pain, or thrombophlebitis. She has taken appetite suppressants at different times. On physical exam, there is jugular venous distention, a palpable right ventricular lift, and a loud P2 hea sound. Chest x-ray shows clear lung fields. ECG shows right axis detion. A perfusion lung scan is normal with no segmental deficits. The most likely diagnosis in this patient is