Multicentric reticulohistiocytosis is a rare systemic disease of unknown aetiology characterized by erosive arthritis and cutaneous lesions consisting of multiple reddish-brown papules and nodules, mainly involving the face and distal upper extremities. It has been suggested that skin eruptions in multicentric reticulohistiocytosis are associated with Köbner phenomenon due to their characteristic distribution, such as on the dorsal aspects of the hands and fingers. We report here a case of a Japanese woman with multicentric reticulohistiocytosis, in whom erythematous macules and papules were widely distributed over the face, ears, neck and the V-area of the chest. Notably, repeated irradiation of ultraviolet (UV) B on the uninvolved back skin resulted in the induction of erythematous macules with infiltration of reticulohistiocytes, indicating the association of UVB-induced Köbner phenomenon with the development of skin lesions, especially on the sun-exposed area. This is the first known report demonstrating the contribution of UV-light-induced Köbner phenomenon for the development of skin eruptions in patients with multicentric reticulo-histiocytosis.

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