A 62-year-old man is admitted to the hospital with a 3-week history of low-grade fevers, f… — USMLE Step 2 CK

A 62-year-old man is admitted to the hospital with a 3-week history of low-grade fevers, fatigue, and progressive shortness of breath. His medical history is significant for mitral valve prolapse with severe regurgitation. Temperature is $38.4^{\circ}\text{C}$ ( $101.1^{\circ}\text{F}$ ), pulse is $102/\text{min}$ , and blood pressure is $110/65 \text{ mmHg}$ . Cardiac examination reveals a grade 3/6 holosystolic murmur at the apex. Small, painless, erythematous macules are noted on the palms.

Laboratory studies show:
- Leukocyte count: $14,200/\text{mm}^3$
- Hemoglobin: $10.4 \text{ g/dL}$
- Serum creatinine: $2.6 \text{ mg/dL}$ (baseline $0.9 \text{ mg/dL}$ three months ago)

Urinalysis reveals 2+ protein, 15-20 red blood cells/hpf, and the presence of red blood cell casts.

Three sets of blood cultures are drawn, and empiric intravenous vancomycin and cefepime are initiated. Transesophageal echocardiography reveals a $1.4\text{-cm}$ oscillating vegetation on the posterior mitral leaflet.

On hospital day 3, all blood culture bottles grow *Enterococcus faecalis*. Antimicrobial susceptibility testing reveals the following minimum inhibitory concentrations (MIC):
- Ampicillin: $\leq 1 \ \mu\text{g/mL}$ (Susceptible)
- Vancomycin: $1 \ \mu\text{g/mL}$ (Susceptible)
- Gentamicin (high-level testing): Susceptible

The laboratory report includes a standard comment: 'Enterococci are intrinsically resistant to cephalosporins, antistaphylococcal penicillins, and clindamycin in vitro.'

The patient's serum creatinine remains elevated at $2.7 \text{ mg/dL}$ .

Which of the following is the most appropriate definitive pharmacotherapy for this patient?

A
Intravenous ampicillin and gentamicin
B
Intravenous vancomycin and gentamicin
Intravenous ampicillin and ceftriaxoneAnswer
D
Intravenous ampicillin monotherapy
E
Intravenous daptomycin monotherapy

Answer

Intravenous ampicillin and ceftriaxone

The most appropriate definitive pharmacotherapy is intravenous ampicillin and ceftriaxone. Enterococcus faecalis infective endocarditis requires a bactericidal antimicrobial regimen, as monotherapy with ampicillin is only bacteriostatic against enterococci. Historically, bactericidal synergy was achieved by combining ampicillin with an aminoglycoside (e.g., gentamicin). However, this patient has developed severe acute kidney injury (creatinine 2.7 mg/dL) secondary to immune complex glomerulonephritis, rendering aminoglycosides highly contraindicated due to their significant nephrotoxicity. The combination of ampicillin and ceftriaxone has emerged as a first-line, nephron-sparing alternative. Although enterococci are intrinsically resistant to cephalosporins in vitro (as noted on the lab report), the combination is highly synergistic in vivo. Ampicillin saturates Penicillin-Binding Proteins 4 and 5 (PBP4/PBP5), while ceftriaxone saturates PBP2 and PBP3. This dual PBP saturation leads to highly effective bactericidal cell wall breakdown.

Step-by-Step Solution

Analyze the clinical and echocardiographic findings.

The patient has native valve infective endocarditis based on fever, new murmur, Janeway lesions, and a large vegetation on the mitral valve.

Establishes the diagnosis and the requirement for a prolonged bactericidal antimicrobial regimen.

Evaluate laboratory results, specifically renal function and urinalysis.

Identification of acute kidney injury (creatinine elevated to 2.7 mg/dL) with RBC casts, indicating immune complex glomerulonephritis.

Recognizing severe renal impairment is critical as it determines the contraindication of nephrotoxic agents like aminoglycosides.

Interpret microbiological culture and susceptibility data.

The pathogen is Enterococcus faecalis. It is susceptible to ampicillin, vancomycin, and high-level gentamicin.

Guides the transition from broad-spectrum empiric therapy to pathogen-directed definitive therapy.

Select the optimal bactericidal regimen that balances efficacy and safety.

Selection of ampicillin plus ceftriaxone.

This combination provides the necessary bactericidal synergy against E. faecalis while avoiding the nephrotoxicity of gentamicin, overcoming the noted in vitro cephalosporin resistance.

Key Concept

The treatment of Enterococcus faecalis infective endocarditis in patients with significant renal impairment requires synergistic dual beta-lactam therapy (ampicillin plus ceftriaxone) to achieve bactericidal activity while avoiding the nephrotoxicity associated with aminoglycosides.

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