Although the relationship between vitiligo and psoriasis is not very clear cut defined, yet it has been suggested that the decline or absence of melanin might be a predisposing factor for developing scaly erythematous plaques of psoriasis.
Another cause that can produce coexistence between psoriasis and vitiligo is the Koebner isomorphic phenomenon response which is found in both.
I hope it helps. Take care and regards.
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