A 44-year-old male presents with dyspnea and abdominal distension for 6 months, worsening over the past week. Examination reveals dyspnea in semi-recumbent position, jugular venous distension, normal cardiac size, and indistinct apical impulse. Heart rate is 100 bpm with regular rhythm. Heart sounds are diminished, and no murmurs are heard. Few rales are noted at lung bases. Abdomen is distended with liver palpable 4 fingerbreadths below the costal margin and tender; hepatojugular reflux is positive. Ascites sign (+), no lower limb edema. BP 95/80 mmHg. ECG shows low voltage and flattened T waves in precordial leads. The most likely diagnosis is: