A 6-day-old female infant is brought to the emergency department by her parents due to poo… — USMLE Step 2 CK

A $6$ -day-old female infant is brought to the emergency department by her parents due to poor feeding, irritability, and lethargy over the past $24$ hours. She was born at $39$ weeks gestation via uncomplicated vaginal delivery to a mother with negative group B *Streptococcus* screening. On examination, the infant is lethargic with a temperature of $38.6^\circ\text{C}$ ( $101.5^\circ\text{F}$ ). Her anterior fontanelle is full and tense. Skin examination reveals a scattered rash consisting of $2$ -mm yellow pustules on a broad erythematous base over the trunk; the parents state this rash appeared on the second day of life and has been waxing and waning since.

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

Parameter	Result	Reference Range
Appearance	Cloudy	Clear
Leukocytes	$1,200/\text{mm}^3$ ( $85\%$ neutrophils)	$0-30/\text{mm}^3$
Erythrocytes	$10/\text{mm}^3$	$0/\text{mm}^3$
Glucose	$20\text{ mg/dL}$ (Serum: $85\text{ mg/dL}$ )	$\geq 40\text{ mg/dL}$
Protein	$160\text{ mg/dL}$	$15-150\text{ mg/dL}$
Gram stain	Gram-positive bacilli	Negative

The physician immediately initiates empiric treatment with intravenous vancomycin and cefotaxime.

Which of the following is the most appropriate modification to this patient's pharmacotherapy?

Add intravenous ampicillinAnswer
B
Replace cefotaxime with ceftriaxone
C
Add intravenous acyclovir
D
Replace vancomycin with intravenous gentamicin
E
Add intravenous dexamethasone

Answer

Add intravenous ampicillin

The patient has neonatal meningitis caused by *Listeria monocytogenes*, indicated by the presence of Gram-positive bacilli in the CSF. The resident inappropriately started vancomycin and cefotaxime, a regimen typically reserved for older infants (>1 month) to cover *Streptococcus pneumoniae* and *Neisseria meningitidis*. *Listeria* is intrinsically resistant to all cephalosporins (including cefotaxime) due to modified penicillin-binding proteins. Therefore, ampicillin must be added immediately, as it is the primary bactericidal agent against *Listeria*. The infant's rash is erythema toxicum neonatorum, a benign, self-limited condition that requires no targeted intervention.

Step-by-Step Solution

Interpret the clinical presentation and CSF findings.

The neonate has fever, lethargy, a full fontanelle, and CSF showing severe neutrophilic pleocytosis, low glucose, and high protein, confirming acute bacterial meningitis.

Establishing the primary diagnosis is necessary to evaluate the appropriateness of the initiated pharmacotherapy.

Analyze the CSF Gram stain to identify the likely pathogen.

The Gram stain reveals Gram-positive bacilli, which in a neonate is pathognomonic for Listeria monocytogenes.

Treatment must be targeted to the specific organism identified.

Evaluate the current empiric antibiotic regimen against the identified pathogen.

The patient is receiving vancomycin and cefotaxime. Cefotaxime is a cephalosporin. Listeria is intrinsically resistant to all cephalosporins due to modified penicillin-binding proteins.

Identifying gaps in empiric coverage prevents life-threatening treatment failure.

Determine the necessary pharmacological modification.

Ampicillin must be added to the regimen, as it is the drug of choice for Listeria monocytogenes.

Ampicillin provides the essential targeted coverage that the current broad-spectrum regimen lacks.

Evaluate the dermatologic findings to rule out viral etiologies.

The pustular rash on an erythematous base present since day 2 is erythema toxicum neonatorum, a benign condition requiring no treatment. It does not indicate HSV.

Ensures no unnecessary antiviral medications (acyclovir) are added.

Key Concept

Neonatal meningitis caused by Listeria monocytogenes requires treatment with ampicillin due to the organism's intrinsic resistance to all cephalosporins.

Estimated Time:1m 30s

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