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 with a sharply demarcated, raised red area and often fever. It is classically caused by Streptococcus pyogenes, a group A beta-hemolytic streptococcus, which has the properties that allow invasion of the superficial dermis and lymphatic spread, producing the characteristic well-defined border of erysipelas. The other organisms listed are associated with different clinical contexts: Streptococcus agalactiae (group B) more often causes neonatal infections; Streptobacillus moniliformis causes rat-bite fever; and Staphylococcus aureus can cause various skin infections but does not define the classic presentation of erysipelas. Thus, the best answer is Streptococcus pyogenes.