A 37-year-old woman is evaluated for exertional shortness of breath. She has had progressive dyspnea for the last year and for the last 2 months has been unable to walk half a block without stopping to rest. She also describes lightheadedness during exertion. Medical history is insignificant, and vital signs are within normal limits. BMI is 23 kg/m2. Physical examination reveals a 2/6 holosystolic murmur at the lower sternal border that increases with inspiration. Further evaluation indicates no evidence of obstructive or interstitial lung disease or venous thromboembolism. Cardiac catheterization shows mean pulmonary artery pressure 43 mm Hg (normal: 8-20) and pulmonary capillary wedge pressure 9 mm Hg (normal: 6-12). Pharmacotherapy blocking the effects of which of the following substances is most likely to benefit this patient?
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