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Difficulty: MediumGeneral Internal Medicine

A 4444-year-old man is evaluated on the medical unit on the third day of hospitalization for a right thigh infection. He initially presented with an expanding, painful, fluctuant mass on his anterior thigh accompanied by a 1010-cm area of surrounding erythema. Upon admission, he had a temperature of 38.4∘C38.4^\circ\text{C} (101.1∘F101.1^\circ\text{F}) and a leukocyte count of 16,000/mm316,000/\text{mm}^3. An incision and drainage were performed, yielding 35 mL35\text{ mL} of purulent material. He was started on empiric intravenous vancomycin and piperacillin-tazobactam.

Today, the patient is afebrile, and his pain is well-controlled. The erythema has receded significantly, and his leukocyte count is 7,500/mm37,500/\text{mm}^3. The patient is tolerating a regular diet. The purulent fluid culture demonstrates heavy growth of *Staphylococcus aureus*. The isolate has the following susceptibility profile:

- Cefazolin: Susceptible
- Clindamycin: Susceptible
- Oxacillin: Susceptible
- Penicillin: Resistant
- Trimethoprim-sulfamethoxazole: Susceptible
- Vancomycin: Susceptible

Which of the following is the most appropriate next step in this patient's pharmacological management?

  1. Transition to oral cephalexinAnswer
  2. B
    Transition to oral linezolid
  3. C
    Transition to oral ciprofloxacin
  4. D
    Maintain intravenous vancomycin and discontinue piperacillin-tazobactam
  5. E
    Maintain intravenous piperacillin-tazobactam and discontinue vancomycin

Answer

Transition to oral cephalexin is the most appropriate management, as the patient has a methicillin-susceptible Staphylococcus aureus (MSSA) infection and is ready for oral step-down therapy.
The correct answer identifies that this patient has an infection caused by methicillin-susceptible Staphylococcus aureus (MSSA) based on the oxacillin susceptibility. Because he is clinically stable, afebrile, and tolerating oral intake, transitioning to an oral step-down agent is indicated. Oral cephalexin (a first-generation cephalosporin) or dicloxacillin are the preferred narrow-spectrum oral beta-lactams for MSSA. This choice appropriately targets the pathogen while adhering to antimicrobial stewardship principles.

Step-by-Step Solution

1
Interpret the culture and susceptibility results.
The culture shows Staphylococcus aureus that is susceptible to oxacillin.
Oxacillin susceptibility indicates that the organism is methicillin-susceptible Staphylococcus aureus (MSSA), which guides the choice of a targeted, narrow-spectrum antibiotic.
2
Evaluate the patient's clinical stability for step-down therapy.
The patient is afebrile, his erythema is receding, his leukocyte count has normalized, and he is tolerating a regular diet.
Clinical improvement and the ability to take oral medications indicate that the patient is ready to transition from intravenous to oral antibiotics.
3
Select the most appropriate targeted oral agent.
Cephalexin, a first-generation cephalosporin, is selected as the optimal step-down therapy.
Cephalexin provides excellent, narrow-spectrum coverage for MSSA, aligning with antimicrobial stewardship principles to minimize collateral damage and resistance.

Key Concept

Patients with methicillin-susceptible Staphylococcus aureus (MSSA) infections who are clinically improving and tolerating oral intake should be stepped down to a narrow-spectrum oral beta-lactam, such as cephalexin or dicloxacillin.
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Topics

General Internal Medicine

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