A case of mycosis fungoides diagnosed in a patient who showed treatment failure with biologics for psoriasis

Mycosis fungoides can be present with scaly erythematous patches or plaques, mimicking psoriasis, especially in early stages with subtle histopathologic findings.Herein, we report a 53-year-old man with a 12-year history of generalized skin lesions.He had been diagnosed with biopsy-proven severe psoriasis vulgaris and treated with guselkumab and other immunosuppressants at an outside hospital.He presented to our hospital with persistent symptoms and did not respond to deucravacitinib.Repeat biopsy was performed to exclude mycosis fungoides.Biopsy specimen showed parakeratosis, psoriasiform epidermal hyperplasia, epidermotropism, and band-like lymphocytic infiltrate.The final diagnosis of mycosis fungoides was made based on clinicopathologic correlation.He was treated with narrowband UVB and topical corticosteroids and showed clinical improvement.In our case, mycosis fungoides was diagnosed in a patient who showed treatment failure with biologics for psoriasis.Mycosis fungoides should be considered in patients with psoriasis unresponsive to biologics, and further evaluation is necessary.