Which organism is the etiologic agent of erysipelas?

• A. Staphylococcus aureus
• B. Streptobacillus moniliformis
• C. Streptococcus agalactiae
• D. Streptococcus pyogenes

Answer: (D)

Erysipelas is a superficial skin infection of the upper dermis and lymphatics, and it is classically caused by Streptococcus pyogenes, a Group A beta-hemolytic streptococcus. This organism readily invades through minor breaks in the skin and spreads via the lymphatics, producing the hallmark presentation of erysipelas: a rapidly spreading, sharp, raised, red plaque with clearly defined borders, often accompanied by fever. Streptococcus pyogenes has virulence factors like M protein that help with adhesion and immune evasion, reinforcing its ability to invade superficial skin and cause this pattern of disease. The other organisms listed are associated with different infections—for example, Staphylococcus aureus more often causes cellulitis without as sharp borders, Streptococcus agalactiae (Group B) is a common neonatal pathogen, and Streptobacillus moniliformis is linked to rat-bite fever rather than erysipelas. Therefore, the organism most characteristically responsible for erysipelas is Streptococcus pyogenes.