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Difficulty: MediumPharmacologic Interventions

A 45-year-old woman is evaluated in an urgent care clinic for a painful skin lesion on her left lower extremity. She states that she noticed a small "spider bite" three days ago that has progressively enlarged and become exquisitely tender. Her past medical history is notable for major depressive disorder, which is well-controlled on fluoxetine. She was also hospitalized six months ago for severe *Clostridioides difficile* colitis after receiving a course of antibiotics for sinusitis. Her temperature is 37.8°C (100.0°F), blood pressure is 122/78 mm Hg, and pulse is 88/min. Physical examination of the left calf reveals a 4-cm fluctuant nodule surrounded by a 6-cm area of intense erythema, induration, and warmth. Bedside ultrasound confirms a localized fluid collection. An incision and drainage procedure is performed, yielding 20 mL of purulent exudate. Given the significant surrounding cellulitis and the high local prevalence of community-acquired methicillin-resistant *Staphylococcus aureus* (MRSA), the physician decides to initiate empiric oral antibiotic therapy. Which of the following is the most appropriate pharmacologic intervention?

  1. A
    Amoxicillin-clavulanate
  2. B
    Linezolid
  3. Trimethoprim-sulfamethoxazoleAnswer
  4. D
    Clindamycin
  5. E
    Cephalexin

Answer

Trimethoprim-sulfamethoxazole is the most appropriate choice because it effectively covers community-acquired MRSA while avoiding critical patient-specific risks (interaction with fluoxetine and a history of severe C. difficile).
The correct intervention is Trimethoprim-sulfamethoxazole. For a patient presenting with a purulent skin and soft tissue infection and significant surrounding cellulitis in an area with high MRSA prevalence, empiric oral MRSA coverage is indicated in addition to incision and drainage. Trimethoprim-sulfamethoxazole provides excellent CA-MRSA coverage while avoiding critical patient-specific contraindications: it does not interact with her SSRI (fluoxetine) and carries a comparatively lower risk of precipitating recurrent Clostridioides difficile.

Step-by-Step Solution

1
Identify the primary clinical indication.
The patient has a purulent skin and soft tissue infection with significant surrounding cellulitis, requiring empiric antibiotic coverage for community-acquired MRSA following incision and drainage.
Guidelines recommend systemic MRSA coverage for purulent infections accompanied by systemic signs or significant surrounding cellulitis.
2
Evaluate potential therapeutic options against patient-specific risk factors.
The patient's history includes active fluoxetine use and a recent severe C. difficile infection.
Comorbidities and current medications must be screened to prevent adverse drug events and iatrogenic harm.
3
Select the safest, most effective medication.
Trimethoprim-sulfamethoxazole covers MRSA and avoids both the serotonin syndrome risk of linezolid and the elevated C. difficile recurrence risk of clindamycin.
It fulfills the microbiological requirement while safely navigating the patient's pharmacologic contraindications.

Key Concept

Empiric antibiotic selection must integrate patient-specific risks, such as avoiding linezolid in patients on SSRIs (risk of serotonin syndrome) and avoiding clindamycin in patients with a history of severe Clostridioides difficile colitis.
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Topics

Pharmacologic Interventions

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