In a leg infection presenting with redness and pain and no significant swelling, which organism is the most common cause of cellulitis?

• A. Streptococcus pyogenes
• B. Pseudomonas aeruginosa
• C. Staphylococcus aureus
• D. Escherichia coli

Answer: (C)

Nonpurulent cellulitis is most often caused by Streptococcus pyogenes (Group A Streptococcus). This organism tends to produce diffuse redness, warmth, and tenderness without focal pus, spreading through the dermis and subcutaneous tissue. In a leg infection with redness and pain but no significant swelling or purulence, this pattern fits Streptococcus pyogenes best. Staphylococcus aureus is more commonly linked to purulent infections, abscesses, or wounds with drainage, while Pseudomonas and Escherichia coli are less typical causes of uncomplicated cellulitis unless there are specific risk factors or wound contexts. Therefore, the most likely organism in this scenario is Streptococcus pyogenes.