A 25 year old man presents to the emergency room with severe abdominal pain. Guaiac test on stool demonstrates occult blood. The patient is taken to exploratory laparotomy, at which a small area of infarcted small bowel is found and surgically removed. Histologic studies on the removed section of bowel demonstrate a recent thrombus occluding a small muscular aery. The adjacent vessel wall shows fibrinoid necrosis with a mixed inflammatory infiltrate containing neutrophils, eosinophils, and mononuclear cells. Which of the following is the most likely cause of the small bowel infarction?