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Difficulty: HardGeneral Internal Medicine

A 68-year-old man is evaluated in the hospital on day 4 of admission for persistent low-grade fevers and fatigue. He originally presented with a 2-week history of generalized weakness, night sweats, and shortness of breath. His medical history is significant for hypertension and osteoarthritis. On examination today, temperature is 37.8°C (100.0°F), blood pressure is 128/76 mmHg, and pulse is 88/min. Cardiac auscultation reveals a new grade 3/6 holosystolic murmur heard best at the cardiac apex. Laboratory studies show a leukocyte count of 12,400/mm³ and serum creatinine of 1.4 mg/dL (baseline 0.9 mg/dL). Blood cultures drawn at admission have grown *Enterococcus faecalis* in 4 out of 4 bottles. Susceptibility testing demonstrates the isolate is susceptible to ampicillin, vancomycin, and linezolid, and lacks high-level aminoglycoside resistance. A transesophageal echocardiogram shows a 1.2-cm mobile vegetation on the mitral valve. The patient is currently receiving empiric therapy with intravenous vancomycin and cefepime. Which of the following is the most appropriate next step in the pharmacologic management of this patient?

  1. Discontinue current antibiotics and initiate intravenous ampicillin and ceftriaxoneAnswer
  2. B
    Continue both intravenous vancomycin and cefepime
  3. C
    Discontinue cefepime and continue intravenous vancomycin monotherapy
  4. D
    Discontinue current antibiotics and initiate intravenous daptomycin
  5. E
    Discontinue cefepime and add intravenous gentamicin to the vancomycin regimen

Answer

The correct step is to discontinue the broad-spectrum empiric antibiotics and initiate targeted therapy with intravenous ampicillin and ceftriaxone.
The most appropriate management is transitioning to intravenous ampicillin and ceftriaxone. *Enterococcus faecalis* endocarditis requires prolonged, bactericidal combination therapy. When the isolate is susceptible, ampicillin is the preferred beta-lactam backbone. Because ampicillin alone is only bacteriostatic against *Enterococcus*, synergy is required to achieve bactericidal activity for deep-seated infections. This synergy is traditionally achieved by adding an aminoglycoside (e.g., gentamicin) or a third-generation cephalosporin (ceftriaxone). In a patient with acute kidney injury (evidenced by a rising serum creatinine from 0.9 to 1.4 mg/dL), the ampicillin and ceftriaxone combination is highly preferred due to comparable efficacy and a significantly lower risk of nephrotoxicity. Discontinuing the empiric vancomycin and cefepime correctly follows antibiotic stewardship protocols by narrowing the spectrum of coverage.

Step-by-Step Solution

1
Interpret the clinical presentation, imaging, and laboratory data.
The patient has native-valve *Enterococcus faecalis* infective endocarditis, complicated by a mild acute kidney injury (elevated creatinine).
Four positive blood cultures, a new regurgitant murmur, and a valvular vegetation on echocardiography confirm the diagnosis. The rising creatinine establishes the renal impairment.
2
Determine the optimal antibiotic backbone based on susceptibility and stewardship principles.
Ampicillin must be selected as the primary agent.
De-escalating from broad-spectrum empiric therapy (vancomycin) to a targeted, narrow-spectrum agent (ampicillin) for susceptible organisms is a foundational rule of antibiotic stewardship.
3
Identify the need for bactericidal synergy and select the safest secondary agent.
Ceftriaxone is added to ampicillin instead of an aminoglycoside.
Ampicillin is only bacteriostatic against *Enterococcus*; endocarditis requires bactericidal synergy. While gentamicin provides synergy, ceftriaxone provides equal synergistic efficacy with significantly less nephrotoxicity, making it the drug of choice in patients with existing renal impairment.

Key Concept

De-escalation of empiric antibiotics to targeted synergistic therapy (ampicillin and ceftriaxone) for *Enterococcus faecalis* endocarditis, prioritizing nephron-sparing regimens in the setting of acute kidney injury.
Estimated Time:1m 30s
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General Internal Medicine

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