A 72-year-old woman complains of shortness of breath on exertion. She states that she also becomes short of breath at night unless she uses three pillows to prop herself up (orthopnea). Physical examination reveals mild obesity, bilateral pitting leg edema, an enlarged liver, and fine crackling sounds on inspiration (rales). A chest x-ray shows cardiomegaly. What is the most likely cause of orthopnea in this patient?