A 67-year-old retired clerk presents with complaints of fati…

A 67-year-old retired clerk presents with complaints of fatigue, shortness of breath, and weight gain over a 2-week period. A nonproductive cough accompanies her symptoms. She reports that climbing up the stairs worsens her dyspnea. The lung exam is positive for fine crackles in the lower lobes with no wheezing. An S3 heart gallop is noted.Which of the following conditions is most likely?