A 37-year-old woman complains of marked general weakness, edema of the face and hands, rapid fatigability when walking, difficult swallowing, disturbances of the cardiac performance. These signs appeared 11 days after a vacation at the sea side. Objectively, the patient has face erythema, "glasses" sign, edema of the lower leg muscles. Heart sounds are muffled, blood pressure is 100/70 mm Hg. In the blood: ASAT -95U, ALAT -130U, increased activity of aldolase and creatine phosphokinase. What examination will be the most informative in this case?