A 52-year-old man from rural Maharashtra presents with acute onset fever, chills, and a rapidly spreading erythematous plaque on his left lower leg. On examination, the lesion is raised, warm, tender, and edematous. The feature marked **B** in the diagram — the sharp, distinct border separating the inflamed skin from normal skin — is a hallmark finding. Which of the following best explains why erysipelas characteristically presents with this sharply demarcated border, in contrast to cellulitis?

Explanation

## Why option 1 is correct Erysipelas is a superficial cellulitis that involves the **upper dermis and superficial lymphatics**, creating a well-demarcated, raised, sharply bordered erythematous plaque. This anatomical limitation to superficial layers produces the characteristic sharp border. In contrast, cellulitis extends into the deeper subcutaneous tissues and fascia, allowing more diffuse spread and resulting in less well-defined, feathered margins. This distinction is the key clinical differentiator between the two conditions and directly reflects the pathophysiology of erysipelas as described in Harrison 21e Ch 130. ## Why each distractor is wrong - **Option 2**: While Streptococcus pyogenes is indeed the most common cause (~80%) of erysipelas, the sharp border is not due to hyaluronidase preventing spread—hyaluronidase actually facilitates bacterial spread through tissues. The border is determined by the anatomical plane of infection (superficial vs. deep), not by enzyme production. - **Option 3**: Although portal of entry (tinea pedis, eczema, wounds) is an important risk factor for erysipelas, the presence of a portal of entry does not explain the sharp border. Cellulitis also occurs with portals of entry but lacks the demarcation because it involves deeper tissues. - **Option 4**: Erysipelas is not limited to the epidermis alone; it involves the **upper dermis and superficial lymphatics**. The sharp border is not at the dermal-epidermal junction but rather reflects the boundary between superficially infected skin and unaffected deeper tissues. Cellulitis also involves the dermis but extends deeper into subcutaneous tissue. **High-Yield:** Erysipelas = superficial (upper dermis + superficial lymphatics) → sharp border; Cellulitis = deep (subcutaneous) → ill-defined border. Same organisms, different depth = different clinical appearance. [cite: Harrison 21e Ch 130; Bailey & Love 28e]