A 47-year-old woman is seen with a 1-wk history of progressive shoness of breath, increasing pedal edema, weight loss, and low-grade fever. She has a 40-pack-year smoking history. Physical examination: pulse 138 bpm; respirations 34/min; blood pressure 100/88 mm Hg with pulsus paradoxus of 22 mm. Peinent findings: increased jugular venous distension, normal sinus rhythm with distant hea sounds, and an apex beat that is difficult to palpate. Chest x-ray is shown below.What is the most likely diagnosis?