A 42-year-old woman with a 3-year history of ulcerative colitis presents with a rapidly enlarging, exquisitely painful ulcer on her left shin. On examination, the lesion marked **A** in the diagram shows a distinctive violaceous, undermined, overhanging border with a purulent necrotic base. Tissue culture is negative for bacterial, fungal, and mycobacterial pathogens. Which of the following best describes the pathophysiological mechanism underlying this condition?