A 73-year-old woman has had episodes of chest pain during the past week. She is afebrile. Her pulse is 80/min, respirations are 16/min, and blood pressure is 110/70 mm Hg. On auscultation of the chest, heart sounds seem distant, but the lung fields are clear. Neck veins are distended to the angle of the jaw, even while sitting. There is a darkly pigmented, irregular, 1.2-cm skin lesion on the right shoulder. A chest radiograph shows prominent borders on the left and right sides of the heart. Pericardiocentesis yields bloody fluid. Laboratory findings include a serum troponin I level of 0.3 ng/mL. Which of the following lesions is the most likely cause of these findings?