Erysipelas is an infection of which skin layer and is most commonly caused by which organism?

• A. Upper dermis; Streptococcus pyogenes
• B. Deep dermis; Staphylococcus aureus
• C. Epidermis only; Pseudomonas
• D. Subcutaneous tissue; Clostridium perfringens

Answer: (A)

Erysipelas is an acute bacterial infection that involves the superficial layers of the skin, specifically the upper dermis and the overlying lymphatics. This superficial involvement explains its hallmark: a raised, sharply demarcated erythematous plaque that often appears with fever and malaise. The organism most commonly responsible is Streptococcus pyogenes, a group A beta-hemolytic streptococcus, which tends to invade the superficial dermal lymphatics and produce the characteristic crisp borders.

Why other options don’t fit: involvement limited to the epidermis would be more like impetigo rather than erysipelas; deeper involvement of the dermis or subcutaneous tissue points to cellulitis rather than erysipelas. Staphylococcus aureus is a common cause of cellulitis and abscesses rather than the classic erysipelas pattern. Pseudomonas and Clostridium species cause other clinical pictures (such as folliculitis or gas gangrene, respectively) and are not the typical culprits for erysipelas.