A 20-year-old female had sore throat, runny nose, and cough 2 weeks ago, which improved with treatment for a cold. Over the past week, she developed palpitations and chest tightness, and experienced syncope 2 days ago, prompting hospitalization. Examination findings: BP 100/70 mmHg (13.3/9.3 kPa), heart rate 40 bpm, enlarged cardiac borders, 2/6 systolic murmur at the apex, no abnormal lung findings. ECG shows Qr pattern in leads II, III, aVF with ST-segment elevation of 0.1 mV, ventricular rate 40 bpm, atrioventricular dissociation (P waves and QRS complexes are unrelated, P wave rate > QRS rate). Lab results: CPK 400 U/L, CPK-MB 40 U/L. The most likely diagnosis is: