A 22-year-old female presents with low-grade fever for 1 month, accompanied by palpitations, shortness of breath, and lower extremity edema for 1 week. Physical exam reveals BP 90/70 mmHg, jugular venous distension, bilateral cardiac enlargement, faint heart sounds, hepatomegaly (2 cm below costal margin), and bilateral lower extremity edema. Echocardiogram shows a 1.0 cm pericardial effusion, and X-ray reveals cardiomegaly. The most likely diagnosis is: