Erysipelas Sharply Demarcated Facial

A 68-year-old woman with type 2 diabetes (HbA1c 8.4%), chronic venous insufficiency, and lower-limb lymphoedema presents with 2 days of a rapidly enlarging painful red rash over her right cheek and nose, high-grade fever (39.1°C), rigors, and malaise. On examination, she has a bright-red, tense, raised, hot, tender plaque with a well-defined advancing edge that steps down sharply onto normal skin in a butterfly distribution across the nose and cheeks. The structure marked **A** in the diagram—the sharply demarcated raised erythematous plaque with step-off edge—is the hallmark clinical finding. Which of the following best describes the pathophysiological basis for this distinctive sharp demarcation in erysipelas?