I correspond with someone who believes that candida overgrowth is the root
cause of psoriasis, though genetic factors surely play a role too.  Candida
is typically associated with a leaky gut, which leads to a sensitivty to
gluten and other proteins; naturally, malabsorption would also be a problem.
I'm not certain that this is always true, but it might be worth getting a
candida antibody test performed.  I tutor a girl with psoriasis (which she's
had since infancy; she was also chronically tired as a child) and am trying
to convince her to get the test done (but it's awkard to ask someone to
shell out money in order to satisfy my curiosity).

-- Aaron Wieland