A 35-year-old woman has experienced palpitations, shortness of breath, poor appetite, and edema for 5 years following exertion, with worsening symptoms over the past 2 weeks after an upper respiratory infection. Examination reveals: BP 120/70 mmHg, cardiomegaly, a diastolic rumbling murmur at the apex, heart rate 120 bpm with irregular rhythm and variable heart sounds, jugular venous distension, bilateral basal crackles, liver 3 cm below the costal margin with tenderness (+), no palpable spleen, and lower extremity edema (+). The pulse rate is 80 bpm. The clinical presentation is most consistent with: