A full-term female infant is examined in the newborn nursery at 48\text{ hours} after birt… — USMLE Step 2 CK

A full-term female infant is examined in the newborn nursery at $48\text{ hours}$ after birth due to the sudden appearance of a widespread skin eruption. She was born at $40\text{ weeks}$ of gestation via uncomplicated vaginal delivery to a $26\text{-year-old}$ primigravid mother. Maternal prenatal laboratory testing was unremarkable, and group B \textit{Streptococcus} screening was negative. Birth weight was $3,450\text{ g}$ . Apgar scores were $7$ and $9$ at $1$ and $5\text{ minutes}$ , respectively. The infant has been breastfeeding well every $2$ to $3\text{ hours}$ and has passed meconium and urine normally.

Vital signs are temperature $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), heart rate $135/\text{min}$ , and respiratory rate $42/\text{min}$ . Physical examination reveals an active, alert neonate. There is a disseminated rash characterized by multiple $1$ to $3\text{ mm}$ pale-yellow pustules surrounded by broad erythematous macules. The lesions are prominent on the trunk, face, and proximal extremities, but completely spare the palms and soles. The remainder of the physical examination, including neurologic reflexes, is within normal limits.

A complete blood count and microscopic evaluation of the pustular fluid are obtained. The results are as follows:

Parameter	Result	Reference Range
Leukocytes	$14,500/\mu\text{L}$	$9,000-30,000/\mu\text{L}$
Neutrophils	$48\%$	$40-60\%$
Lymphocytes	$35\%$	$20-40\%$
Eosinophils	$12\%$	$1-3\%$
Pustule Wright-Giemsa stain	Predominance of bilobed granular leukocytes; no microorganisms seen	-

Which of the following is the most appropriate next step in the management of this patient?

Reassurance and continued routine newborn careAnswer
B
Blood, urine, and cerebrospinal fluid cultures followed by intravenous ampicillin and gentamicin
C
Empirical administration of intravenous acyclovir and surface swabbing for polymerase chain reaction
D
Application of topical mupirocin ointment to the affected skin areas
E
Skin scraping for potassium hydroxide preparation and empirical topical nystatin

Answer

Reassurance and continued routine newborn care

The correct answer is reassurance and continued routine newborn care. This neonate presents with classic findings of erythema toxicum neonatorum (ETN), which is one of the most common benign rashes in healthy term newborns. It typically appears within the first 24 to 72 hours of life. The rash is characterized by firm, 1 to 3 mm pale-yellow or white papules and pustules surrounded by a prominent erythematous base (often described as having a 'flea-bitten' appearance). The lesions frequently appear on the trunk and proximal extremities but characteristically spare the palms and soles. The underlying pathophysiology involves a sterile, localized accumulation of eosinophils. This is definitively demonstrated in this vignette by the peripheral eosinophilia (12%) and the Wright-Giemsa stain of the pustule contents, which reveals bilobed granular leukocytes (eosinophils) and lacks microorganisms. Because ETN is an entirely benign condition that spontaneously resolves without scarring over 1 to 2 weeks, no laboratory evaluation or pharmacological intervention is required. The most appropriate management is to reassure the parents and continue routine newborn care.

Step-by-Step Solution

Analyze the patient's clinical presentation and physical examination findings.

The patient is a healthy, full-term neonate at 48 hours of life with normal vital signs and a rash composed of pustules on an erythematous base that spares the palms and soles.

Identifying the timing, morphology, and distribution of the rash alongside the patient's systemic stability is essential to differentiate between benign newborn rashes and life-threatening congenital or neonatal infections.

Interpret the laboratory and cytological data.

The peripheral blood shows eosinophilia (12%), and the Wright-Giemsa stain of the pustule reveals bilobed granular leukocytes (eosinophils) without any bacteria or viral cytopathic effects.

Cytology provides the definitive pathophysiologic mechanism for the rash, ruling out bacterial or viral etiologies.

Synthesize the findings to establish a diagnosis.

The combination of a 'flea-bitten' pustular rash sparing the palms/soles in a healthy neonate, coupled with eosinophils in the pustules and blood, is diagnostic of erythema toxicum neonatorum (ETN).

ETN is a common, self-limited, immune-mediated condition characterized by localized eosinophilic infiltration.

Determine the appropriate management plan based on the diagnosis.

Because ETN is entirely benign and resolves spontaneously within 1 to 2 weeks, the correct step is reassurance and observation.

Recognizing benign conditions prevents unnecessary, painful, and potentially harmful invasive procedures and antibiotic use.

Key Concept

Erythema toxicum neonatorum is a benign, self-limited rash appearing in the first few days of life, characterized by erythematous macules with central pustules containing eosinophils, requiring only reassurance.

Estimated Time:1m 30s

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