Although a difficult diagnosis to make, pulmonary arterial hypertension (PAH) is the most likely diagnosis in this young woman who has used appetite suppressants. Pulmonary arterial hypertension (also known as primary pulmonary hypertension) has been associated with fenfluramine-type appetite suppressants. The predominant symptom is dyspnea, which is usually not apparent until the disease has advanced. When physical findings, chest x-ray, or echocardiography suggest pulmonary hypertension, recurrent pulmonary emboli must be ruled out. In this case, a normal perfusion lung scan makes pulmonary angiography unnecessary. Right-to-left cardiac shunts cause hypoxia (oxygen desaturation) that characteristically does not improve with oxygen supplementation. Restrictive lung disease should be ruled out with pulmonary function testing but is unlikely with a normal chest x-ray. An echocardiogram will show right ventricular enlargement and a reduction in the left ventricle size consistent with right ventricular pressure overload. Left ventricular dysfunction can cause pulmonary edema but not pulmonary hypertension.