I don't know if the skin problem I had this winter was actually psoriasis or something else, but it developed in spite of the 10g of vitamin C I take a day (although the C helps greatly with other issues). I was also taking 1000 IU of vitamin D since the fall and that didn't make any difference either. Then I read more on D3 and upped my dose of D3 to 6000IU a day for a while and suddenly the crunchy, scaly sections of skin have cleared and softened. I take lots of other supplements, so I can't prove it's the D3, but that's the only real change I've made recently. 6000IU may be too much to take ongoing, I'm trying to figure out what's right. I'll stop supplementing altogether in the summer and just spend time outside in the sun.
UV light treatment is also used for psoriasis, maybe that works because UV/B stimulates production of vitamin D in the skin?