A 16-day-old male infant is brought to the emergency department after experiencing a gener… — USMLE Step 2 CK

A 16-day-old male infant is brought to the emergency department after experiencing a generalized tonic-clonic seizure lasting approximately two minutes. The parents report that he has been increasingly irritable and feeding poorly for the past 24 hours. The patient was born at 38 weeks of gestation via an uncomplicated vaginal delivery. Maternal prenatal screening was unremarkable, including a negative rectovaginal culture for Group B *Streptococcus* at 36 weeks.

On physical examination, the infant is lethargic with a rectal temperature of $38.8^\circ\text{C}$ ( $101.8^\circ\text{F}$ ), heart rate of $175/\text{min}$ , and respiratory rate of $55/\text{min}$ . He has a full, tense anterior fontanelle. A sparse, non-pruritic, erythematous papular eruption with central pustules is noted on the trunk.

A lumbar puncture is performed, and the cerebrospinal fluid (CSF) analysis reveals the following:

Parameter	Patient Result	Reference Range
Appearance	Cloudy	Clear
Leukocyte count	$1,200/\text{mm}^3$	$0-30/\text{mm}^3$
Neutrophils	$85\%$	$0-6\%$
Glucose	$15\text{ mg/dL}$	$40-80\text{ mg/dL}$
Protein	$180\text{ mg/dL}$	$15-45\text{ mg/dL}$

The patient is promptly started on empiric antimicrobial therapy. Forty-eight hours later, the laboratory reports that the CSF culture is growing a non-spore-forming, Gram-positive bacillus that exhibits tumbling motility at room temperature.

Based on these findings, which of the following is the most appropriate targeted pharmacotherapy for this patient?

Intravenous ampicillin and gentamicinAnswer
B
Intravenous cefotaxime monotherapy
C
Intravenous vancomycin and ceftriaxone
D
Intravenous acyclovir and ampicillin
E
Intravenous penicillin G monotherapy

Answer

Intravenous ampicillin and gentamicin

The correct answer is intravenous ampicillin and gentamicin. This patient has bacterial meningitis caused by *Listeria monocytogenes*, as evidenced by the cerebrospinal fluid (CSF) analysis and the classic microbiologic description of a non-spore-forming, Gram-positive bacillus exhibiting tumbling motility. While empiric therapy for neonatal meningitis typically includes ampicillin and either a third-generation cephalosporin (such as cefotaxime) or an aminoglycoside, therapy must be optimized and narrowed once the pathogen is definitively identified. *Listeria* species possess altered penicillin-binding proteins, making them intrinsically resistant to all cephalosporins. Therefore, agents like cefotaxime and ceftriaxone are ineffective for targeted treatment. Although *Listeria* is susceptible to penicillin G and ampicillin, beta-lactam monotherapy is merely bacteriostatic against this organism. In cases of severe infection within a privileged site, such as meningitis or endocarditis, bactericidal activity is strictly required. This is achieved by combining ampicillin with an aminoglycoside, such as gentamicin, which provides the necessary synergistic bactericidal effects. Finally, the patient's trunk rash is consistent with erythema toxicum neonatorum, a benign physiologic condition that requires no targeted intervention.

Step-by-Step Solution

Analyze the clinical presentation and cerebrospinal fluid (CSF) findings.

The patient has a severe central nervous system infection, specifically bacterial meningitis, indicated by hypoglycorrhachia, elevated protein, and marked neutrophilic pleocytosis.

Establishing the correct syndrome narrows the diagnostic and therapeutic focus.

Identify the causative pathogen based on the microbiologic description.

The organism is identified as Listeria monocytogenes, which is classically characterized as a non-spore-forming, Gram-positive bacillus with tumbling motility.

Targeted pharmacotherapy requires accurate identification of the specific etiology to ensure susceptibility.

Evaluate the erythematous papular eruption on the trunk.

The rash represents erythema toxicum neonatorum, a benign physiologic finding, rather than a manifestation of herpes simplex virus (HSV).

Correctly distinguishing benign rashes prevents unnecessary and potentially toxic empirical therapies, such as acyclovir.

Determine the optimal antimicrobial regimen for Listeria meningitis.

Intravenous ampicillin and gentamicin is selected.

Listeria exhibits intrinsic resistance to cephalosporins. Ampicillin is the drug of choice, and the addition of gentamicin provides essential bactericidal synergy for central nervous system infections.

Key Concept

Targeted pharmacotherapy for Listeria monocytogenes neonatal meningitis requires ampicillin combined with an aminoglycoside for bactericidal synergy, as the organism is intrinsically resistant to all cephalosporins.

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