Dermatology MCQ - Viral Infections - Eruptive hypomelanosis

A 5-year-old child presents with the sudden appearance of numerous, small, asymptomatic, hypopigmented macules on the trunk and proximal extremities. The lesions are uniformly 2-4 mm in diameter and are not scaly. The child is otherwise well and had a mild upper respiratory infection two weeks prior. Which of the following is the most likely diagnosis and its presumed etiology?

A) Eruptive hypomelanosis; viral infection
B) Pityriasis alba; eczema-associated hypopigmentation
C) Tinea versicolor; Malassezia yeast overgrowth
D) Vitiligo; autoimmune melanocyte destruction
E) Postinflammatory hypopigmentation; preceding inflammation

Correct Answer: A) Eruptive hypomelanosis; viral infection

Explanation

This presentation is classic for eruptive hypomelanosis (EH), a distinct viral exanthem characterized by sudden-onset hypopigmentation.

Key Clinical Features of Eruptive Hypomelanosis:

• Lesion Description: Multiple, small (2-4 mm), round to oval, hypopigmented macules. They are non-scaly and asymptomatic.

• Distribution: Primarily on the trunk and proximal extremities; the face is often spared.

• Onset: Sudden eruption over a few days.

• Symptoms: The child is afebrile and otherwise well. No pruritus or preceding inflammation.

• Preceding Illness: Often follows a viral upper respiratory infection (e.g., picornavirus, adenovirus).

• Course: Lesions resolve spontaneously within 3-4 weeks with repigmentation.

Presumed Etiology:

• Viral Infection: EH is considered a post-viral exanthem. Viral particles or an immune response to infection may temporarily disrupt melanocyte function or melanin transfer, leading to transient hypopigmentation.

• It is often grouped with other viral exanthems like eruptive pseudoangiomatosis.

Why Not the Other Options?

• (B) Pityriasis alba: Presents with ill-defined, hypopigmented, slightly scaly patches on the face and arms, often in the context of atopic dermatitis. It is not eruptive or sudden in onset.

• (C) Tinea versicolor: Causes hypopigmented (or hyperpigmented) macules with fine scale on the trunk. It is due to Malassezia overgrowth and is not associated with a recent viral illness.

• (D) Vitiligo: Features sharply demarcated, complete depigmented macules (white, not off-white) that are permanent and often expand over time. It is autoimmune and not eruptive.

• (E) Postinflammatory hypopigmentation: Occurs at sites of previous inflammation (e.g., eczema, psoriasis), which is not reported here. The lesions are not uniform or eruptive.

Management:

• Reassurance: No treatment is needed, as EH is self-resolving.

• Sun protection: To avoid contrast between affected and normal skin.

Prognosis:
Excellent; complete repigmentation occurs within weeks to months.

Note: Eruptive hypomelanosis is a benign condition that must be distinguished from other hypopigmentary disorders. The key clues are the sudden onset in a well child, uniform small macules, lack of scale, and history of a recent viral infection. It is a diagnosis of exclusion, but the clinical presentation is highly characteristic.