A 25-year-old previously healthy woman has had worsening fatigue with dyspnea, palpitations, and fever over the past week. On physical examination her vital signs show T 38.9 C, P 103/minute, R 30/minute, and BP 95/65 mm Hg. Her heart rate is slightly irregular. An ECG shows diffuse ST-T segment changes. A chest x-ray shows mild cardiomegaly. An echocardiogram shows slight mitral and tricuspid regurgitation but no valvular vegetations. Laboratory studies show a troponin I of 12 ng/mL. She recovers over the next two weeks with no apparent sequelae. Which of the following laboratory test findings best explains the underlying etiology for these events?

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