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

A 29-year-old woman is evaluated for a 5-day history of nodules over her lower extremities. She reports that she regularly visits a local spa that uses whirlpool footbaths during her pedicure procedures; she always shaves her legs with a razor before these visits. Medical history is unremarkable, and she takes no medications.

On physical examination, temperature is 37.0 °C (98.6 °F), blood pressure is 120/70 mm Hg, pulse rate is 70/min, and respiration rate is 14/min. BMI is 22. No inguinal lymphadenopathy is apparent. Several mildly tender, erythematous nodules and papules are noted over the distal lower extremities bilaterally; several lesions appear furuncular with associated ulceration. The remainder of the examination is normal.

Punch biopsy of a lesion reveals a necrotizing granulomatous dermatitis, and tissue culture grows a mycobacterial species within 4 days.

Which of the following is the most likely cause of infection?

A. Mycobacterium fortuitum
B. Mycobacterium gordonae
C. Mycobacterium kansasii
D. Mycobacterium marinum

MKSAP Answer and Critique

The correct answer is A: Mycobacterium fortuitum.

This patient has Mycobacterium fortuitum–associated skin infection. Nontuberculous mycobacteria can colonize salon water, and M. fortuitum furunculosis has been well described in patients who obtain pedicures at nail salons that use contaminated whirlpool footbaths. Recent leg shaving with a razor increases the risk for these infections, likely owing to resultant skin nicking. The distal lower-extremity skin lesions, which usually arise 3 to 4 weeks after inoculation of the organism, appear in areas that have been exposed to spa water. Initially papular, these progress to a boil before ulcerating. The fast growth of the organism in culture (<7 days) is consistent with rapidly growing mycobacteria, such as M. fortuitum, Mycobacterium chelonae, or Mycobacterium abscessus.

Mycobacterium gordonae, Mycobacterium kansasii, and Mycobacterium marinum are slowly growing bacteria (that is, growth on solid media requires more than 7 days and usually 2 to 4 weeks). M. gordonae is a ubiquitous organism that rarely causes infection in immunocompetent hosts. Isolation of the organism in patients who are not immunosuppressed usually reflects colonization or contamination.

An environmental pathogen, M. kansasii typically causes a lung infection that mimics tuberculosis. Risk factors include COPD, HIV, alcohol abuse, exogenous immunosuppression, and cancer. Skin infection is uncommon and usually occurs in immunosuppressed patients with disseminated disease.

M. marinum can cause trauma-associated skin infection after exposure to fresh or salt water, including fish tanks and swimming pools. Initially papular before ulcerating, skin lesions are sometimes referred to as “fish tank granuloma.” The rapid growth of the culture from this patient’s lesions (<7 days) makes M. marinum unlikely.

Key Point

• Mycobacterium fortuitum furunculosis is a well-described skin infection in patients who obtain pedicures at nail salons that use contaminated whirlpool footbaths.

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