A 71-year-old man is evaluated on hospital day 3 for ongoing management of acute pyeloneph… — USMLE Step 2 CK

A $71$ -year-old man is evaluated on hospital day $3$ for ongoing management of acute pyelonephritis and concurrent bacteremia. At the time of admission, he presented with a temperature of $39.4^\circ\text{C}$ ( $102.9^\circ\text{F}$ ), heart rate of $112 \text{ /min}$ , and blood pressure of $88/54 \text{ mm Hg}$ . Following aggressive fluid resuscitation, he was empirically started on intravenous ceftriaxone. He has demonstrated significant clinical improvement, currently reporting minimal flank pain. Today, his temperature is $37.5^\circ\text{C}$ ( $99.5^\circ\text{F}$ ) and blood pressure is $122/78 \text{ mm Hg}$ . Blood and urine cultures obtained prior to antibiotic initiation both demonstrate heavy growth of *Enterobacter cloacae*. The microbiology laboratory reports the following minimum inhibitory concentrations (MIC) and interpretations for the isolate:

Antimicrobial Agent	Interpretation
Ampicillin	Resistant
Cefazolin	Resistant
Ceftriaxone	Susceptible
Cefepime	Susceptible
Piperacillin-tazobactam	Susceptible
Meropenem	Susceptible

Which of the following is the most appropriate next step in pharmacotherapy?

Switch to intravenous cefepimeAnswer
B
Continue intravenous ceftriaxone
C
Switch to intravenous meropenem
D
Switch to intravenous piperacillin-tazobactam
E
Add oral clavulanate to the current regimen

Answer

The most appropriate next step is to switch to intravenous cefepime.

The correct approach involves recognizing that *Enterobacter cloacae* produces an inducible chromosomal AmpC beta-lactamase. Third-generation cephalosporins (like ceftriaxone) act as strong inducers of this enzyme and are subsequently hydrolyzed by it, leading to treatment failure even when in vitro testing reports them as susceptible. Cefepime, a fourth-generation cephalosporin, is uniquely stable against AmpC hydrolysis and is a very weak inducer of the enzyme. According to current infectious disease guidelines, cefepime is the definitive drug of choice for invasive AmpC-producing infections (like bacteremia) that are susceptible to it.

Step-by-Step Solution

Identify the pathogen's intrinsic resistance profile.

*Enterobacter cloacae* is recognized as a SPACE organism carrying an inducible chromosomal AmpC beta-lactamase gene.

Understanding the pathogen's resistance mechanisms is critical because in vitro susceptibility does not always predict in vivo efficacy.

Evaluate the current empiric therapy (ceftriaxone) against the pathogen's profile.

Although ceftriaxone tests as 'Susceptible' in vitro, it is a strong inducer of AmpC and is easily hydrolyzed by it.

Continuing a third-generation cephalosporin for an AmpC producer risks selecting for derepressed mutant strains, leading to inevitable clinical failure.

Select the optimal definitive therapy that balances efficacy and antimicrobial stewardship.

Cefepime is chosen over meropenem and piperacillin-tazobactam.

Cefepime (a fourth-generation cephalosporin) is highly stable against AmpC enzymes and is preferred over carbapenems to prevent the emergence of multi-drug resistant organisms.

Key Concept

Management of invasive infections caused by AmpC-producing Enterobacterales (e.g., *Enterobacter cloacae*) requires transitioning from third-generation cephalosporins to cefepime to prevent inducible resistance, even when in vitro testing suggests broader susceptibility.

Estimated Time:1m 30s

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