Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.

A 21-year-old woman is evaluated for mosquito bites on her arms and legs that she received 1 week ago that she has been scratching regularly. One of the bites on her left thigh is now painful with a small amount of drainage. She otherwise feels well, has no significant medical history, and takes no medications.

On physical examination, vital signs are normal. A weeping red papule with overlying honey-colored crust is present on the anterior left thigh. There is no surrounding or extension of the redness, no lymphadenopathy, and no systemic symptoms. The remainder of the examination is unremarkable.

Which of the following is the most appropriate treatment?

A. Cephalexin
B. Doxycycline
C. Mupirocin ointment
D. Triamcinolone ointment

MKSAP Answer and Critique

The correct answer is C. Mupirocin ointment.

The most appropriate treatment for this patient is mupirocin ointment for localized impetigo. Impetigo is a superficial bacterial infection most commonly caused by Staphylococcus aureus and, to a lesser extent, β-hemolytic streptococci. The patient initially had an insect bite that became secondarily infected, likely due to the skin trauma associated with scratching the bite.

Because there are no signs of systemic illness and the patient has a localized infection, treatment with topical antibacterial agents, such as mupirocin, is considered first-line therapy.

Bleach baths may be a treatment option (put 1/4 to 1/2 cup of common liquid bleach into the bath water to create a chlorinated bath), which decreases colonization of S. aureus). Topical retapamulin cream also has been demonstrated to have effective antimicrobial properties against Staphylococcus and Streptococcus spp. and can be effective in patients with mupirocin-resistant organisms.

Although cephalexin can be effective for impetigo, localized infections should be treated with topical antibiotics whenever possible to avoid systemic side effects and potential resistance with overuse of oral antibiotics. If the infection was systemic or more severe, cephalexin would be appropriate. In addition, if topical mupirocin failed to resolve the infection, oral therapy can be considered.

Doxycycline is effective against methicillin-resistant Staphylococcus aureus but provides poor coverage against Streptococcus, which is another organism that can lead to impetigo. In addition, topical therapy is considered first-line therapy for localized impetigo.

Triamcinolone ointment is not effective against impetigo. Triamcinolone ointment can be used for symptomatic treatment of insect bites; however, when pain and honey-colored crust is present, impetigo needs to be considered and treated.

Key Point

• A topical antibacterial agent, such as mupirocin, is the first-line therapy for localized impetigo.

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