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Difficulty: HardPediatric Infectious Diseases

An 8-year-old boy is evaluated in the hospital on hospital day 4. He was originally admitted for acute, severe right thigh pain, limp, and high fever. Magnetic resonance imaging (MRI) of the right lower extremity at admission demonstrated marrow edema and a subperiosteal fluid collection in the distal femur. He was diagnosed with acute hematogenous osteomyelitis and empirically started on intravenous vancomycin and ceftriaxone. Over the last 48 hours, his temperature has normalized, and his thigh pain has significantly decreased.

Blood cultures drawn prior to the administration of antibiotics have grown *Staphylococcus aureus*. The susceptibility report is as follows:

AntimicrobialInterpretation
AmpicillinResistant
CefazolinSusceptible
ClindamycinSusceptible
OxacillinSusceptible
VancomycinSusceptible

The patient has no known drug allergies. Which of the following is the most appropriate next step in the pharmacologic management of this patient?

  1. Discontinue vancomycin and ceftriaxone, and initiate intravenous cefazolinAnswer
  2. B
    Continue intravenous vancomycin and ceftriaxone to complete a 4-week course
  3. C
    Discontinue ceftriaxone and continue intravenous vancomycin as monotherapy
  4. D
    Discontinue vancomycin and ceftriaxone, and initiate intravenous ceftazidime
  5. E
    Discontinue vancomycin and ceftriaxone, and initiate oral amoxicillin

Answer

The most appropriate next step is to discontinue vancomycin and ceftriaxone, and initiate intravenous cefazolin.
The patient has acute hematogenous osteomyelitis caused by Methicillin-Susceptible Staphylococcus aureus (MSSA), confirmed by the isolate's susceptibility to oxacillin and cefazolin. In cases of invasive MSSA infection, targeted beta-lactam antibiotics—such as first-generation cephalosporins (cefazolin) or anti-staphylococcal penicillins (oxacillin, nafcillin)—are the standard of care. They possess superior bactericidal activity, excellent bone penetration, and are associated with lower rates of relapse and mortality compared to vancomycin. Therefore, the most appropriate step in antimicrobial stewardship and clinical management is to discontinue the broad-spectrum empiric regimen (vancomycin and ceftriaxone) and de-escalate to intravenous cefazolin.

Step-by-Step Solution

1
Analyze the culture and sensitivity data.
The pathogen is Staphylococcus aureus. It is susceptible to oxacillin and cefazolin, but resistant to ampicillin.
Oxacillin susceptibility officially classifies the organism as Methicillin-Susceptible Staphylococcus aureus (MSSA). Ampicillin resistance is due to staphylococcal penicillinase.
2
Evaluate the current empiric antibiotic regimen.
The patient is on vancomycin (covering MRSA) and ceftriaxone (covering Gram-negatives and some Gram-positives).
Empiric therapy is intentionally broad to cover potential MRSA, Kingella kingae, and other pathogens pending culture results.
3
Apply principles of antimicrobial stewardship and efficacy to select targeted therapy.
Vancomycin and ceftriaxone should be discontinued. Intravenous cefazolin (or an anti-staphylococcal penicillin like oxacillin) should be initiated.
For invasive MSSA infections like osteomyelitis, first-generation cephalosporins (cefazolin) or anti-staphylococcal penicillins are definitively superior to vancomycin, offering faster bactericidal activity, better bone penetration, and lower rates of clinical failure.

Key Concept

For invasive Methicillin-Susceptible Staphylococcus aureus (MSSA) infections, narrow-spectrum beta-lactams (e.g., cefazolin, oxacillin) are the drugs of choice and are clinically superior to vancomycin.
Estimated Time:1m 30s
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Pediatric Infectious Diseases

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