A 45-year-old man presents to his primary care physician with a rash. He first noticed the rash 2 weeks prior and he endorses intermittent itchiness from the rash. His medical history is significant for chronic hepatitis C infection, hypertension, type 2 diabetes mellitus, and heart failure. The patient works as a butcher at a local meat processing shop and is sexually active with several partners. He does not use barrier protection. His temperature is 98.6°F (37.0°C), pulse is 90/min, blood pressure is 155/95 mmHg, and respirations are 12/min. On physical exam, his skin has the finding shown in Figure A. Fine, lace-like white lines are also noted on the patient’s buccal mucosa. Which of the following is the most likely diagnosis?
Atopic dermatitis
Lichen planus
Scabies
Psoriasis
Lichen planus
This patient presents with several purple, polygonal, pruritic papules and plaques. These findings are most consistent with a diagnosis of lichen planus.

Lichen planus is an inflammatory skin disorder most commonly characterized by an eruption of Purple, Polygonal, Planar, Pruritic, Papules and Plaques (the 6 P’s), often on extensor surfaces such as the wrist. Development of fine white lines on the surfaces of skin lesions, as well as on the buccal mucosa (Wickham striae), are also characteristically seen in this disease. Concurrent hepatitis C infection is a risk factor for lichen planus. A skin biopsy will show a sawtooth infiltrate of lymphocytes at the dermal-epidermal junction. Treatment is with antihistamines for pruritus and topical steroids for localized disease as well as addressing the underlying cause.

Tziotzios et al. review the evidence regarding the diagnosis and treatment of lichen planus. They discuss how the molecular basis of this disorder is relatively unknown. They recommend making a clear diagnosis to allow for treatment.

Figure/Illustration A shows multiple purple, polygonal, papules and plaques on the wrist (red circles). These findings are characteristic of lichen planus.

Incorrect Answers:
Answer A: Atopic dermatitis most commonly presents with pruritic, lichenified plaques on the flexural surfaces. The mucous membranes are not involved. Treatment is with topical moisturizers and antihistamines for pruritus. This disease is also associated with other atopic conditions such as asthma and seasonal allergies.

Answer C: Prurigo nodularis is characterized by hypertrophic, dome-shaped papules that develop secondary to chronic scratching. The mucous membranes are not involved. Treatment is with avoidance of the triggering behavior such as scratching and antihistamines for pruritus.

Answer D: Psoriasis is characterized by silver, scaly plaques that bleed easily when the scale is physically removed (Auspitz sign). Patients with cutaneous psoriasis are at increased risk for the development of psoriatic arthritis. Psoriasis can be treated with topical corticosteroids as well as calcipotriene.

Answer E: Scabies is a cutaneous infection with the mite Sarcoptes scabei that presents with severe pruritus, especially in the intertriginous areas. Patients may have serpiginous scaling lesions that represent the burrowing of the mite through the skin, but mucous membrane lesions are absent. Treatment can include topical permethrin first line.

Bullet Summary:
Lichen planus presents with the 6 P’s – Purple, Pruritic, Polygonal, Planar, Papules and Plaques.