Cellulitis

A 58-year-old man with chronic venous insufficiency and poorly controlled type 2 diabetes mellitus presents with a 48-hour history of progressive erythema, warmth, pain, and swelling of the right lower extremity. He has low-grade fevers and recalls a small portal of entry at an area of dry cracked interdigital skin with early tinea pedis. On examination, the right lower extremity reveals diffuse erythema extending from the dorsum of the foot to the mid-calf. The structure marked **A** in the diagram—poorly demarcated warm erythema with ill-defined borders that fade gradually into normal-appearing skin without a sharp raised edge—is characteristic of which clinical condition?