A 35-year-old woman complains of slowly progressive dyspnea. Her history is otherwise negative, and there is no cough, sputum production, pleuritic chest pain, or thrombophlebitis. She has taken appetite suppressants at different times. Physical examination reveals jugular venous distention, a palpable right ventricular lift, and a loud P2 heart sound. Chest x-ray shows clear lung fields. Oxygen saturation is 94%. ECG shows right axis deviation. A perfusion lung scan is normal, with no segmental deficits. Which of the following is the most likely diagnosis?