For several weeks, a 35-year-old man has had worsening chest pain, frontal headache, and diffuse muscle and joint pain. The patient was initially treated conservatively with ibuprofen(Drug information on ibuprofen) for a presumed viral syndrome. However, because of his minimal response to the NSAID and a strong family history of early cardiac disease (his father had a "heart attack" at age 40), he is hospitalized.HISTORY
The patient was previously healthy; he takes no long-term medications. He does not smoke or use illicit substances. He works as a professional arborist in southeastern Pennsylvania and frequently spends time in areas of heavy foliage. However, he has been unable to work regularly since the onset of his symptoms.PHYSICAL EXAMINATION
Temperature is 37.2°C (99°F); heart rate, 52 beats per minute; blood pressure, 110/60 mm Hg; and oxygen saturation, normal on room air. No rashes, lymphadenopathy, or oral lesions are noted. Aside from bradycardia, results of a cardiac examination are normal. The lungs are clear, and the abdomen is normal. There is diffuse joint tenderness with palpation and deep inspiration, especially along the costosternal joints; no effusion is noted. Results of a neurologic examination are nonfocal.LABORATORY AND IMAGING RESULTS
Results of a complete blood cell count and serum chemistry panel are all normal. Serum total cholesterol level is 133 mg/dL. Erythrocyte sedimentation rate is 54 mm/h. Cardiac troponin levels are less than 0.05 ng/mL on admission (normal range, 0.05 to 0.5), and at 8 and 16 hours thereafter.
Chest radiograph shows no active disease. ECG reveals first-degree atrioventricular (AV) block, with a PR interval of 0.32 millisecond; otherwise it is normal. A transthoracic echocardiogram shows normal left ventricular function with no pericardial effusion. Myocardial perfusion imaging reveals no evidence of ischemia; however, the maximum heart rate with exercise is only 95 beats per minute (target rate, 157 beats per minute). During the exercise portion of the study, frequent episodes of Mobitz type I and type II conduction delays are noted; these continue intermittently on telemetry for several hours afterward.Which of the following studies is most likely to lead to the diagnosis?
A. Antistreptolysin O titers.
B. Coronary angiography.
C. Enzyme-linked immunosorbent assay (ELISA) and Western blot testing for Borrelia burgdorferi.
D. Genetic testing for long QT syndrome.