A 43-year-old woman presents to the clinic with a 6-month history of progressive shortness of breath, especially with exertion. She also reports occasional fatigue and lightheadedness during physical activity but denies chest pain or syncope. Physical examination reveals mild jugular venous distention, a loud pulmonary component of the second heart sound (P2), and peripheral edema. She is being evaluated for possible pulmonary hypertension. Of the following, which is the most common symptom of pulmonary hypertension?
