An 18-month-old girl presented with a skin eruption that began 1 month earlier. The child's mother reported that 2 weeks before the eruption, the patient had an upper respiratory tract infection but has been asymptomatic since the rash's onset. Over-the-counter hydrocortisone cream has been ineffective.

Physical examination revealed linear plaques of small, palpable, hypopigmented papules on the patient's dorsolateral left arm (Figure 1). No lymphadenopathy was noted.

QUESTION

Based on the patient's history and physical examination, which one of the following is the most likely diagnosis?

• A. Incontinentia pigmenti.

• B. Lichen striatus.

• C. Linear epidermal nevus.

• D. Linear morphea.

DISCUSSION

The correct answer is B: lichen striatus, which is consistent with the patient's presentation of small, unilateral papules along the Blaschko lines. Although lichen striatus is usually a straightforward clinical diagnosis, several presentations are possible and can vary based on skin tone. The condition typically occurs in children 5 months to 5 years of age and manifests unilaterally on the extremities, but it can appear on the trunk or, in rare cases, on the fingernails.¹ Patients are usually otherwise asymptomatic, but the papules may be pruritic. Lichen striatus typically presents with a linear configuration, but color and palpability can vary. Although the papules are usually erythematous, this can appear more subtle in patients with darker skin tones, making the diagnosis more challenging.