Dermatology MCQ - Viral Infections - Asymmetric Periflexural Exanthem of Childhood

A 4-year-old child presents with a unilateral, erythematous, blanchable maculopapular rash that began in the axilla and is now spreading to the trunk and proximal limbs. The rash is non-pruritic, and the child is afebrile and otherwise well. The mother notes that a sibling had a similar rash a few weeks prior. Which of the following is the most likely diagnosis and its characteristic historical feature?

A) Asymmetric periflexural exanthem of childhood; spontaneous resolution
B) Gianotti-Crosti syndrome; association with hepatitis B
C) Papular urticaria; history of insect bites
D) Pityriasis rosea; herald patch
E) Scabies; nocturnal pruritus

Correct Answer: A) Asymmetric periflexural exanthem of childhood; spontaneous resolution

Explanation

This presentation is classic for asymmetric periflexural exanthem of childhood (APEC), also known as unilateral laterothoracic exanthem.

Key Clinical Features of APEC:

• Initial Presentation: The rash often begins unilaterally, typically in the axilla or groin.

• Spread: It may spread to the trunk and proximal limbs but often retains a asymmetric distribution.

• Morphology: Erythematous, blanchable macules and papules that may become confluent. It is often non-pruritic or only mildly so.

• Course: The rash evolves over days to weeks and is followed by spontaneous resolution without scaling or sequelae within 3-6 weeks.

• Epidemiology: Common in young children (ages 1-5 years). Household spread to siblings is a recognized feature, suggesting an infectious, likely viral, etiology.

Why Not the Other Options?

• (B) Gianotti-Crosti syndrome: Presents with symmetric, monomorphic, flat-topped papules on the face, extremities, and buttocks. It is associated with hepatitis viremia, not unilateral axillary onset.

• (C) Papular urticaria: Presents as pruritic papules (often with central puncta) in groups or clusters, triggered by insect bites. It is not unilateral and lacks the characteristic periflexural onset and spread.

• (D) Pityriasis rosea: Typically begins with a herald patch followed by a bilateral, symmetric "Christmas tree" distribution on the trunk. It often affects older children and adults.

• (E) Scabies: Causes intensely pruritic papules, burrows, and vesicles in web spaces, wrists, and genitals. Nocturnal pruritus is hallmark. It is not typically unilateral or periflexural in onset.

Management:

• Supportive care: Reassurance is primary, as the condition is self-limiting.

• No specific treatment is required. Topical emollients or mild corticosteroids may be used for minimal pruritus.

Prognosis:
Excellent; resolves completely without scarring. Recurrence is rare.

Note: APEC is a common, benign exanthem of childhood. Its unilateral onset and periflexural location are highly characteristic. The history of a similar rash in a sibling supports its suspected infectious (viral) etiology. Always consider this diagnosis in a young child with a unilateral, non-toxic presentation.