A 45-year-old woman presents with a 2-day history of fever, pain, and swelling of the left lower leg. On examination, there is a bright red, warm, tender area with marked edema. The structure marked **B** in the diagram—the sharp, distinct border between affected and unaffected skin—is a key clinical feature that helps differentiate this condition from cellulitis. Which of the following best explains the pathophysiological basis for this sharply demarcated border in erysipelas?

Explanation

## Why "Involvement of superficial dermis and superficial lymphatics with clear demarcation at the dermal-epidermal junction" is right Erysipelas is a superficial cellulitis that affects the upper dermis and superficial lymphatics, creating a characteristic sharply demarcated, raised erythematous plaque. The sharp distinct border marked **B** reflects the anatomical limitation of the infection to the superficial dermal layer, with a clear boundary at the dermal-epidermal junction. This superficial involvement contrasts with cellulitis, which extends into deeper dermis and subcutaneous tissue, resulting in poorly defined, indistinct borders. The lymphatic involvement in erysipelas also contributes to the well-demarcated appearance. (Harrison 21e, Ch 130) ## Why each distractor is wrong - **Deep involvement of subcutaneous tissue with poorly defined borders due to fascial spread**: This describes cellulitis, not erysipelas. Cellulitis involves deeper layers and lacks the sharp border characteristic of erysipelas. - **Involvement of deeper dermis and subcutaneous tissue with gradual transition to normal skin**: Again, this is cellulitis pathology. The gradual transition and deeper involvement are opposite to the superficial, sharply demarcated nature of erysipelas. - **Involvement of muscle fascia with rapid spread along tissue planes and indistinct margins**: This describes necrotizing fasciitis, a surgical emergency with rapid progression and systemic toxicity—not the superficial, well-demarcated erysipelas. **High-Yield:** Erysipelas = superficial cellulitis (upper dermis + superficial lymphatics) with SHARP borders; Cellulitis = deeper dermis/subcutaneous with INDISTINCT borders; Necrotizing fasciitis = muscle fascia involvement with SEVERE PAIN OUT OF PROPORTION and surgical emergency. [cite: Harrison 21e Ch 130]