Impetigo is most commonly caused by which organism?

• A. Erysipelothrix rhusiopathiae
• B. Staphylococcus saprophyticus
• C. Streptococcus pyogenes
• D. Corynebacterium diphtheriae

Answer: (C)

Impetigo is a contagious, superficial skin infection whose classic bacterial culprits are Streptococcus pyogenes (group A Streptococcus) and Staphylococcus aureus. Among the organisms listed, Streptococcus pyogenes best fits as the classic cause of impetigo, especially the nonbullous form seen in children. It causes lesions that start as vesicles or papules and quickly crust over with the characteristic honey-colored crusts; transmission is through direct contact, which is why these infections spread in close-contact settings. In terms of lab and clinical clues, Streptococcus pyogenes is a Gram-positive coccus in chains and a beta-hemolytic, Lancefield group A organism, often susceptible to penicillin. The other options are associated with different diseases: Erysipelothrix rhusiopathiae causes erysipeloid from environmental exposure; Corynebacterium diphtheriae causes diphtheria; Staphylococcus saprophyticus is a common urinary pathogen, not a typical cause of impetigo.