Pyoderma Gangrenosum with Inflammatory Bowel Disease

A 38-year-old woman with established ulcerative colitis in moderate flare presents with a tender 4-cm ulcer on her left lower leg that began as a small pustule 12 days ago after a minor knee scrape. The lesion has rapidly expanded with exquisitely painful undermined, violaceous-to-dusky borders overhanging a moist, gray, necrotic base. Bacterial and fungal cultures are negative. Doppler studies exclude vascular etiology. A deep biopsy shows neutrophil-predominant infiltrate with abscess formation but no true vasculitis or granulomas. The structure marked **A** in the diagram represents this lesion. Which of the following best characterizes the pathophysiology of this condition?