Cellulitis Ill-Defined Warmth

A 56-year-old man with type 2 diabetes, venous stasis, and recurrent tinea pedis presents with 3 days of a hot, painful, swollen right calf with low-grade fever and malaise. On examination, the right calf is diffusely erythematous, tender, and swollen. The structure marked **A** in the diagram—the ill-defined erythema with gradually fading border—is the key clinical feature that distinguishes this condition from erysipelas. Which of the following best describes the pathophysiological basis and appropriate initial management for this presentation?