A 35-year-old male presents with chest tightness for the past 6 months, occasionally feeling a heavy pressure on the anterior chest. Over the past month, he reports dyspnea on exertion. Physical exam: Supine position shows prominent jugular vein distension, enlarged cardiac dullness, normal heart sounds, heart rate 98/min (regular), no murmurs. Lungs clear. Liver palpable 2 fingerbreadths below costal margin with positive hepatojugular reflux sign. Echocardiography reveals moderate pericardial effusion with normal chamber sizes. Pericardiocentesis yields 500ml of deep yellow fluid. Laboratory tests: Specific gravity >1.018, protein 45g/L (4.5g/dl), cell count 0.4×10<sup>9</sup>/L (400/μl) with 80% lymphocytes. The most likely diagnosis is: