A 52-year-old woman presented with a moderately pruritic rash on her right leg that developed several months earlier. She did not have blisters, cough, sore throat, chills, diarrhea, fevers, or joint aches. She had no recent infections or household contacts with a similar rash. She was not using any new medications or personal care products. No improvement was noted after two weeks of treatment with oral prednisone.

Physical examination revealed a purple papular eruption extending in a streak pattern from the medial aspect of the right thigh to the posteromedial aspect of the right calf (Figure 1). Close examination revealed fine white lines within purple plaques. Vital signs were normal.

Question

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

• A. Follicular keratosis.

• B. Inflammatory linear verrucous epidermal nevus.

• C. Lichen striatus.

• D. Linear lichen planus.

• E. Linear psoriasis.

Discussion

The answer is D: linear lichen planus. The prevalence of lichen planus is 0.5% to 1%. Linear lichen planus is a rare form of the condition (less than 1% of cases) that usually presents as unilateral pruritic, violaceous, flat-topped papules in a linear distribution on the limbs.¹ Fine white lines on top of the plaques (Wickham striae) may be seen. The underlying cause of lichen planus is unknown, although the linear form may be associated with a dermatomal pattern or the Koebner phenomenon at the site of trauma.² Classic lesions present with the four P's: purple, pruritus, polygonal, and papules/plaques.