A 52-year-old man is evaluated in the hospital on day 3 of admission for severe right lowe… — USMLE Step 2 CK

A 52-year-old man is evaluated in the hospital on day 3 of admission for severe right lower extremity cellulitis and fever. On presentation to the emergency department, his temperature was $39.2^\circ\text{C}$ ( $102.6^\circ\text{F}$ ) and his blood pressure was $92/58\text{ mmHg}$ . Blood cultures were drawn, and he was empirically started on intravenous vancomycin and piperacillin-tazobactam along with fluid resuscitation. Today, his temperature is $37.3^\circ\text{C}$ ( $99.1^\circ\text{F}$ ), his blood pressure is $124/78\text{ mmHg}$ , and the erythema and induration of his right leg have significantly decreased. The admission blood cultures are now growing *Staphylococcus aureus* in both sets. Susceptibility testing demonstrates that the organism is resistant to penicillin, but susceptible to oxacillin, cefazolin, and vancomycin. Which of the following is the most appropriate next step in this patient's pharmacotherapy?

A
Discontinue piperacillin-tazobactam and continue intravenous vancomycin
B
Continue both intravenous vancomycin and piperacillin-tazobactam
Discontinue current antibiotics and start intravenous cefazolinAnswer
D
Discontinue vancomycin and continue intravenous piperacillin-tazobactam
E
Discontinue current antibiotics and start oral cephalexin

Answer

The most appropriate next step is to discontinue the broad-spectrum empiric regimen (vancomycin and piperacillin-tazobactam) and initiate targeted, narrow-spectrum therapy with intravenous cefazolin.

The correct course of action is to de-escalate the patient's empiric antibiotic regimen to a targeted, narrow-spectrum agent. The blood cultures grew methicillin-susceptible Staphylococcus aureus (MSSA), evident from its susceptibility to oxacillin and cefazolin. For MSSA bacteremia, intravenous antistaphylococcal penicillins (e.g., nafcillin, oxacillin) or first-generation cephalosporins (e.g., cefazolin) are the drugs of choice. These beta-lactam antibiotics have been shown to have faster rates of bacterial clearance and lower rates of treatment failure and mortality compared to vancomycin for MSSA infections. Furthermore, discontinuing piperacillin-tazobactam appropriately narrows the spectrum of coverage, removing unnecessary antipseudomonal and anaerobic activity.

Step-by-Step Solution

Interpret the clinical presentation and culture results.

The patient has bacteremia secondary to cellulitis, caused by methicillin-susceptible Staphylococcus aureus (MSSA), as indicated by susceptibility to oxacillin and cefazolin.

Accurate identification of the pathogen and its susceptibility profile is required to tailor antibiotic therapy.

Apply principles of antimicrobial stewardship for MSSA bacteremia.

Empiric broad-spectrum coverage must be de-escalated to a definitive narrow-spectrum agent.

Targeted therapy minimizes toxicity, reduces the risk of Clostridioides difficile infection, and prevents the development of antimicrobial resistance.

Select the optimal agent and route for MSSA bacteremia.

Intravenous beta-lactams like cefazolin or nafcillin/oxacillin are chosen over vancomycin due to superior clinical outcomes.

S. aureus bacteremia carries a high risk of metastatic infection (e.g., endocarditis), requiring aggressive IV therapy with the most bactericidal agent available. Vancomycin has slower bacterial clearance and higher failure rates for MSSA compared to beta-lactams.

Key Concept

Empiric broad-spectrum antibiotics should be de-escalated to targeted, narrow-spectrum intravenous beta-lactams (e.g., cefazolin, nafcillin) for the treatment of methicillin-susceptible Staphylococcus aureus (MSSA) bacteremia.

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