What is the cause of this man's itchy leg rash?

A 21-year-old male presented for evaluation of a rash that started on his legs 1 month prior following a sunburn earlier in the summer. The rash has remained itchy and red. The patient is otherwise healthy and takes no medications. He denies any new exposures. 

What can be the cause of this rash?

A. Cutaneous lupus erythematosus

B. Atopic dermatitis

C. Psoriasis

D. Leukocytoclastic vasculitis

E. Polymorphous light eruption

(Answer and discussion on next page)

Answer: Photodistributed psoriasis

A biopsy confirmed the diagnosis of psoriasis. Most likely, this patient had a Koebner phenomenon or pathergy response to the sunburn injury, causing him to develop psoriasis in the distribution of his sunburn. He has been managed with topical therapy. 

Lupus would be an unusual diagnosis in a young male, but is possible. Atopic dermatitis should have a previous history. Vasculitis would be unlikely and more purpuric. Polymorphous light eruption would not be as persistent and usually on the head and neck.