An infectious diseases consultation is requested for a 72-year-old man due to persistent f… — USMLE Step 2 CK

An infectious diseases consultation is requested for a 72-year-old man due to persistent fevers. He was admitted to the hospital 4 days ago with a 3-week history of malaise, low-grade fevers, and chills. These symptoms began approximately one month after he underwent a cystoscopy for benign prostatic hyperplasia. His medical history is notable for hypertension, type 2 diabetes mellitus, and stage 3 chronic kidney disease.

On physical examination, his temperature is $38.3^\circ\text{C}$ ( $100.9^\circ\text{F}$ ), blood pressure is $138/82\text{ mmHg}$ , and pulse is $92\text{/min}$ . Cardiac auscultation reveals a new grade $2/6$ diastolic decrescendo murmur heard best at the right upper sternal border. Examination of the hands shows small, linear, dark red streaks in the nail beds.

A transesophageal echocardiogram demonstrates a $1.2\text{-cm}$ mobile vegetation on the aortic valve with moderate aortic regurgitation. Four sets of blood cultures drawn upon admission are all positive for *Enterococcus faecalis*.

Antimicrobial susceptibility testing of the isolate shows the following:

Antimicrobial Agent	Interpretation	MIC
Ampicillin	Susceptible	$1.0 \mu\text{g/mL}$
Vancomycin	Susceptible	$2.0 \mu\text{g/mL}$
Linezolid	Susceptible	$1.0 \mu\text{g/mL}$
Gentamicin	High-level resistance	$> 500 \mu\text{g/mL}$
Streptomycin	High-level resistance	$> 1000 \mu\text{g/mL}$

Current laboratory results reveal a leukocyte count of $14,200/\text{mm}^3$ and a serum creatinine of $1.9\text{ mg/dL}$ (baseline $1.8\text{ mg/dL}$ ).

Which of the following is the most appropriate definitive intravenous antimicrobial regimen for this patient?

Ampicillin and ceftriaxoneAnswer
B
Ampicillin and gentamicin
C
Ampicillin monotherapy
D
Vancomycin monotherapy
E
Daptomycin monotherapy

Answer

Ampicillin and ceftriaxone

The correct answer is ampicillin and ceftriaxone. *Enterococcus faecalis* infective endocarditis requires bactericidal therapy to sterilize the cardiac vegetation. Because beta-lactams are generally only bacteriostatic against enterococci, dual therapy is necessary to achieve a synergistic bactericidal effect. The traditional regimen is ampicillin plus an aminoglycoside (e.g., gentamicin). However, this patient's isolate exhibits high-level gentamicin resistance (HLGR), meaning the addition of gentamicin provides no synergistic benefit and only adds unnecessary nephrotoxicity (especially concerning given his underlying chronic kidney disease). In cases of HLGR, the combination of ampicillin and ceftriaxone is the treatment of choice. Ampicillin binds to penicillin-binding proteins 4 and 5 (PBP4, PBP5), while ceftriaxone binds to PBP2 and PBP3. This dual, complementary saturation of different PBPs produces a potent bactericidal effect against *E. faecalis*, overcoming the organism's intrinsic tolerance to cephalosporins. Notably, this dual beta-lactam regimen is now increasingly preferred as first-line therapy even for non-HLGR strains due to its excellent efficacy and lack of nephrotoxicity.

Step-by-Step Solution

Identify the clinical syndrome.

The patient has native valve infective endocarditis caused by Enterococcus faecalis, likely seeded during his recent cystoscopy.

The presence of fever, a new aortic regurgitation murmur, splinter hemorrhages, and positive blood cultures confirm the diagnosis.

Analyze the antimicrobial susceptibility results.

The isolate is susceptible to ampicillin but demonstrates high-level resistance to both gentamicin and streptomycin (HLAR).

Determining the resistance profile is critical for selecting a synergistic bactericidal regimen, which is an absolute requirement for curing endocarditis.

Select the optimal definitive therapy.

Ampicillin combined with ceftriaxone is chosen as the definitive regimen.

Monotherapy is only bacteriostatic against enterococci. While ampicillin plus gentamicin traditionally provides synergy, HLAR renders the aminoglycoside ineffective. Ampicillin plus ceftriaxone provides necessary bactericidal synergy via dual PBP inhibition without relying on aminoglycosides.

Key Concept

The definitive treatment for Enterococcus faecalis infective endocarditis requires synergistic bactericidal therapy; for isolates with high-level aminoglycoside resistance (HLAR), the combination of ampicillin and ceftriaxone is the standard of care.

Estimated Time:1m 30s

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