A 45-year-old man with a history of tinea pedis presents with abrupt onset of fever, chills, and a painful erythematous plaque on his left lower leg. On examination, the lesion shows a brilliantly red, warm, tender area with a sharply demarcated, raised border (marked **B** in the diagram) separating it from adjacent normal skin. Regional lymph nodes are enlarged and tender. Which of the following clinical features is MOST directly explained by the sharply demarcated, raised border characteristic of the structure marked **B**?

Explanation

## Why option 1 is right The sharply demarcated, raised border marked **B** is the hallmark clinical feature that distinguishes erysipelas from cellulitis. This sharp demarcation directly reflects the anatomical level of infection: erysipelas affects the upper dermis and superficial lymphatics, creating a well-defined boundary. In contrast, cellulitis involves the deeper dermis and subcutaneous fat, producing poorly defined, indistinct borders. This distinction is foundational to the diagnosis and management of these two streptococcal skin infections (Robbins 10e Ch 8; Harrison 21e Ch 124). ## Why each distractor is wrong - **Option 2**: Cellulitis (not erysipelas) involves the subcutaneous fat and fascia. The sharply demarcated border of erysipelas actually indicates superficial involvement, NOT deep involvement requiring surgical intervention. Erysipelas is managed medically with antibiotics. - **Option 3**: The causative organism (Group A Streptococcus vs. S. aureus) is not determined by the border characteristics alone. Both erysipelas and cellulitis can be caused by different organisms; the sharp demarcation is a morphological feature reflecting depth of infection, not microbial identity. - **Option 4**: Erysipelas involves the dermis (not epidermis alone) and superficial lymphatics. It is not a purely superficial epidermal condition; the involvement of the upper dermis is what produces the characteristic raised, erythematous plaque. **High-Yield:** Sharp, raised border = superficial (upper dermis + superficial lymphatics) = erysipelas; indistinct, poorly defined border = deep (deeper dermis + subcutaneous fat) = cellulitis. [cite: Robbins 10e Ch 8; Harrison 21e Ch 124]