Erysipelas is caused by which organism?

Correct answer: A. Group A beta-haemolytic Streptococcus (Streptococcus pyogenes)

• A. CORRECT. Erysipelas is a superficial cellulitis of the upper dermis and superficial lymphatics caused overwhelmingly by group A beta-haemolytic Streptococcus (S. pyogenes). Because the infection sits in the superficial dermis and tracks along lymphatics, it produces a lesion with sharply demarcated, raised, advancing borders — the feature that distinguishes it clinically from ordinary cellulitis, whose margins are indistinct. It commonly affects the face and lower legs, and responds to penicillin.
• B. S. aureus is the dominant organism in furuncles, carbuncles, impetigo (along with S. pyogenes) and purulent cellulitis, but it is not the cause of classic erysipelas.
• C. A mixed streptococcal and staphylococcal picture describes ordinary cellulitis or an abscess, not erysipelas. Erysipelas is characteristically a single-organism streptococcal disease.
• D. Pseudomonas aeruginosa causes infections in burns, chronic wounds and immunocompromised hosts — for example ecthyma gangrenosum — not erysipelas.

Note: Distinguish by depth: impetigo is epidermal, erysipelas is upper dermis with a sharp raised border, cellulitis is deeper dermis and subcutis with a diffuse border, and necrotising fasciitis is fascial with pain out of proportion to findings.