An infant is delivered at 34 weeks of gestation to a 28-year-old primigravida. The mother … — USMLE Step 2 CK

An infant is delivered at 34 weeks of gestation to a 28-year-old primigravida. The mother experienced a mild febrile illness with myalgias three weeks prior to delivery, shortly after consuming unpasteurized dairy products at a farm stand. Following birth, the infant initially appears well but develops progressive lethargy, apneic episodes, and temperature instability by 24 hours of life. Vital signs show a temperature of $35.8^\circ\text{C}$ ( $96.4^\circ\text{F}$ ), heart rate of 170/min, and respiratory rate of 65/min. Skin examination reveals scattered 1-2 mm pale yellow papules on an erythematous base over the chest and trunk, sparing the palms and soles. A lumbar puncture is performed, and cerebrospinal fluid analysis demonstrates a leukocyte count of $850/\text{mm}^3$ with $80\%$ neutrophils, glucose of $20\text{ mg/dL}$ , and protein of $150\text{ mg/dL}$ . Gram stain of the fluid reveals intracellular and extracellular Gram-positive bacilli. Which of the following is the most appropriate antimicrobial regimen for this patient?

A
Intravenous acyclovir and ampicillin
B
Intravenous ceftriaxone and vancomycin
Intravenous ampicillin and gentamicinAnswer
D
Intravenous cefotaxime and vancomycin
E
Intravenous meropenem and linezolid

Answer

Intravenous ampicillin and gentamicin

The patient's clinical presentation of neonatal sepsis with meningitis, combined with the maternal history of consuming unpasteurized dairy and a flu-like illness, strongly suggests infection with Listeria monocytogenes. This is confirmed by the CSF Gram stain showing Gram-positive bacilli. Listeria is intrinsically resistant to all cephalosporins. The treatment of choice for Listeria meningitis is intravenous ampicillin, typically combined with an aminoglycoside such as gentamicin for synergistic bactericidal activity. The infant's rash is classic for erythema toxicum neonatorum, a benign condition that does not require intervention.

Step-by-Step Solution

Identify the likely pathogen based on the maternal history and CSF Gram stain.

Maternal consumption of unpasteurized dairy followed by a flu-like illness, combined with neonatal meningitis and Gram-positive bacilli in the CSF, specifically indicates Listeria monocytogenes infection.

Recognizing the precise pathogen allows for the selection of targeted, highly effective antimicrobial therapy.

Interpret the clinical significance of the neonatal rash.

The scattered pale yellow papules on an erythematous base sparing palms and soles is diagnostic of erythema toxicum neonatorum (ETN), a benign self-limiting rash.

Differentiating a benign rash from an infectious exanthem (like HSV) prevents unnecessary antiviral administration.

Select the optimal antimicrobial regimen for the identified pathogen.

Listeria monocytogenes requires treatment with ampicillin, often paired with gentamicin for synergistic bactericidal activity. It is intrinsically resistant to cephalosporins.

Applying knowledge of intrinsic bacterial resistance prevents the administration of ineffective empirical therapies like cefotaxime.

Key Concept

Early-onset neonatal sepsis and meningitis caused by Listeria monocytogenes requires targeted therapy with ampicillin (often with gentamicin), as the organism is intrinsically resistant to all cephalosporins.

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