Erysipelas is caused by which organism?

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

• A. STAPHYLOCOCCUS AUREUS causes deeper cellulitis, furuncles, abscesses and impetigo. Staphylococcal cellulitis has an INDISTINCT, poorly demarcated border — the opposite of erysipelas.
• B. CLOSTRIDIUM PERFRINGENS causes gas gangrene (clostridial myonecrosis), a deep necrotising infection with crepitus and systemic toxicity — a completely different clinical picture.
• C. ALPHA-HAEMOLYTIC (viridans) streptococci are oral commensals and the leading cause of subacute infective endocarditis. They do not cause erysipelas.
• D. CORRECT. ERYSIPELAS is a superficial cellulitis of the upper dermis and superficial lymphatics caused almost exclusively by GROUP A BETA-HAEMOLYTIC STREPTOCOCCUS (Streptococcus pyogenes). It presents as a fiery red, warm, painful plaque with a SHARPLY DEMARCATED, RAISED border that is clearly palpable against normal skin — the face and lower legs are the classic sites. Fever and lymphadenopathy are usual. Penicillin is the drug of choice.

Note: The examinable discriminator is the BORDER: erysipelas is sharply demarcated and raised (superficial, streptococcal); ordinary cellulitis has a diffuse, indistinct edge (deeper, more often staphylococcal). Same genus also gives you scarlet fever, pharyngitis, rheumatic fever and post-streptococcal glomerulonephritis.