MKSAP: 25-year-old woman is evaluated for redness at mosquito bite

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

A 25-year-old woman is evaluated for redness that developed over her right leg at the site of a mosquito bite. She is otherwise healthy and takes no medications.

On physical examination, temperature is 37.2 °C (99.0 °F), blood pressure is 120/70 mm Hg, pulse rate is 70/min, and respiration rate is 14/min. There is an erythematous 3 × 3-cm patch on the right thigh. The area is warm to the touch with no evidence of purulence, fluctuance, crepitus, or lymphadenopathy.

Which of the following is the most appropriate empiric outpatient therapy?

A: Cephalexin
B: Doxycycline
C: Fluconazole
D: Metronidazole
E: Trimethoprim-sulfamethoxazole

MKSAP Answer and Critique

The correct answer is A: Cephalexin.

This patient has nonpurulent cellulitis that is most likely caused by β-hemolytic streptococci, and empiric outpatient treatment with a β-lactam agent such as cephalexin or dicloxacillin is recommended. Cellulitis is a bacterial skin infection involving the dermis and subcutaneous tissues. This infection is most frequently associated with dermatologic conditions involving breaks in the skin, such as eczema, tinea pedis, or chronic skin ulcers, and conditions leading to chronic lymphedema, such as mastectomy and lymph node dissections or saphenous vein grafts used in bypass surgery. Cellulitis should be suspected in patients with the acute onset of spreading erythema, edema, pain or tenderness, and warmth. Fever, although common, is not uniformly present. Patients with severe disease may have associated systemic toxicity. The most common pathogens are Staphylococcus aureus and the β-hemolytic streptococci, especially group A β-hemolytic streptococci (GABHS). GABHS is most often associated with nonpurulent cellulitis, whereas S. aureus may cause concomitant abscesses, furuncles, carbuncles, and bullous impetigo.

Doxycycline and trimethoprim-sulfamethoxazole have activity against community-associated methicillin-resistant S. aureus but are not reliably effective against β-hemolytic streptococci.

Fluconazole is an antifungal agent. Fungi do not usually cause cellulitis in young, healthy persons, but fungal infection should be considered in immunocompromised patients.

Metronidazole is an antimicrobial agent used to treat some anaerobic bacterial and protozoal infections. Although metronidazole is active against some microaerophilic bacteria, it is not effective for treatment of β-hemolytic streptococci.

Key Point

  • Outpatients with nonpurulent cellulitis should be treated empirically with a β-lactam agent such as cephalexin or dicloxacillin that is active against β-hemolytic streptococci.

This content is excerpted from MKSAP 16 with permission from the American College of Physicians (ACP). Use is restricted in the same manner as that defined in the MKSAP 16 Digital license agreement. This material should never be used as a substitute for clinical judgment and does not represent an official position of ACP. All content is licensed to on an “AS IS” basis without any warranty of any nature. The publisher, ACP, shall not be liable for any damage or loss of any kind arising out of or resulting from use of content, regardless of whether such liability is based in tort, contract or otherwise.

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