A 35-year-old male presents with exertional dyspnea, palpitations, shortness of breath, oliguria, and lower extremity edema for over 1 year. Symptoms worsened 1 week ago after sore throat, cough with yellow sputum, and dyspnea at night (orthopnea). Echocardiogram shows dilation of both ventricles, diffuse hypokinesis, and left ventricular ejection fraction (LVEF) of 30%. He takes digoxin 0.25mg once daily with no significant past medical history. The most likely cause is: