An unusual rash on the feet

A 51 year old white homosexual man was admitted with cellulitis of the left leg. His medical history included psoriasis, for which he had received six years of methotrexate. The methotrexate was stopped 11 years before this presentation and he had not experienced any active psoriasis since then. Examination revealed multiple painless, indurated violaceous plaques and verrucous growths on the dorsal and plantar aspects of both feet (fig 1). Further questioning revealed that these lesions had been present for more than 10 months. There was no evidence of psoriasis on examination. What investigation would you request?