A 53-year-old man has a worsening exertional dyspnea for 2 days, for 2 days. He has had an upper respiratory illness 3 weeks ago, but the symptoms never resolved completely. He has no chest pain or palpitation. His BP is 87/60 mm Hg and pulse is 109/min. Oxygen saturation is 95 % on room air. His body mass index is 23kg/m2. Internal jugular venous pulsation is 11cm, lungs are clear to auscultation, heart sounds are muffled. Which of the following is the most likely cause of this patient's complaints?