I wonder (assuming that Prurisol does work well on moderate psoriasis), if people who are treated with it would never progress to the severe stage. Or, if some people will progress to severe no matter what.

There are some diseases that are problematic to treat because lots of different diseases with similar symptoms are lumped into one category.There are diseases that have similar symptoms to other diseases, so knowing WHICH disease it is determines what the correct treatment is (Remember the TV show "House"?) What if moderate psoriasis and severe psoriasis are actually two different diseases?

Some people think that's one of the problem with Alzheimer's drugs -- the patient population isn't separated out into different groups because they have similar symptoms, but it's theorized that they may actually be different diseases with different causes. Autism used to be one broad category, now they are dividing it into all kinds of sub-groups.

So, it will be interesting to see if P works at a certain stage (moderate) but not later stage, or if there are dosage issues (later stage needs more, or a longer time period?)

One thing to remember is that this really is a pretty short period of treatment. It takes about 6 weeks for a skin cell to make it from the bottom-most layer out to the surface of the skin, so it might be that people with more severe disease just need to take it for more than one skin turn-over cycle.