USMLE PRACTICE QUESTION OF THE WEEK

Question for December 5, 2025

A 7-year-old boy is brought to the clinic with a 4‑day history of pruritic sores around his mouth and nose. Several classmates have similar rashes. He is afebrile and playful. Exam reveals multiple well‑demarcated erythematous plaques with honey‑colored crusts on the perioral skin; no mucosal involvement or lymphadenopathy is present.

Which of the following is the most likely causative organism?

Correct Answer:

C. Staphylococcus aureus

Correct Answer Explanation:

Nonbullous impetigo presents with vesicles or pustules that rupture to form honey-colored crusts, most commonly caused by Staphylococcus aureus (and less often Streptococcus pyogenes). Viral etiologies (HSV, VZV) produce grouped vesicles or diffuse lesions with different evolution and distribution. Candida or dermatophytes produce scaling erythema or annular lesions rather than crusted erosions.

Further Insight:

  • Topical mupirocin is first-line for limited disease; oral agents for extensive lesions or outbreaks.
  • Treat household contacts with hygiene measures to reduce spread.