A 42-year-old man with poorly controlled diabetes presents with acute onset of warmth, erythema, and tenderness over his left lower leg. On examination, the erythematous area shows **diffuse, poorly defined borders** (marked **A** in the diagram). There is associated fever (38.5°C), regional lymphadenopathy, and a red streak extending proximally along the leg. A diagnosis of cellulitis is made. Which of the following clinical features best distinguishes this condition from erysipelas?