(for short answer, please see the last page) Psoriasis is an autoimmune skin condition which leads to an overproduction of skin cells called keratinocytes. The immune system mistakes components of the body's own skin as pathogens, such as a virus or bacteria, that should be eradicated from the body. This autoimmune reaction leads to inflammation and an overproduction of skin cells, resulting in scaling, itch and pain.
The endocannabinoid system is the body's own naturally occurring network of receptors and endogenous cannabinoids which, among other vital bodily functions, is involved in immunoregulation. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3044336/ Several studies have shown that the endocannabinoid system is intricately involved in downregulating an overactive and/or self-attacking immune reaction. This knowledge has led to breakthroughs in treating conditions such as MS, traumatic brain injury and Parkinson's disease. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3044336/ .
So, it's not much of a surprise that several studies suggest that both endocannabinoids and phytocannabinoids can directly treat the symptoms of psoriasis. However, please keep in mind that these are pre-clinical studies, i.e. not performed on humans. http://www.jaad.org/article/S0190-9622(17)30308-0/fulltext https://jamanetwork.com/journals/jama/fullarticle/2338256
In fact, the phytocannabinoids THC, CBD, CBN and CBG have all been found to inhibit keratinocyte proliferation in hyper-proliferating keratinocyte cell lines, such as those found in psoriasis. http://www.jaad.org/article/S0190-9622(17)30308-0/fulltext
Considering the above, I recommend that you seek "whole plant" products, such as the ones offered by Care By Design and Kiva Confections. Most cannabis products specifically isolate the THC and/or CBD. However, whole plant products should also contain other cannabinoids, such as the aformentioned CBN and CBG, as well as terpenes.
"Whole plant therapeutics also have a wider window of efficacy and are less likely to trigger side effects than isolates. In other words, they’re safer, less likely to cause side effects, and they work better... for a startling array of medical conditions. This is why we’re wholly committed to full spectrum, whole plant medicines." https://www.cbd.org/products
"We practice a WHOLE PLANT PHILOSOPHY, which means we don't only extract the CBD. We extract all of the other plant materials so all its compounds and cannabinoids work together for best results" http://kivaconfections.com/products/bars/ginger http://kivaconfections.com/products/bars/espresso
For psoriasis, and most issues which topical cannabis may be of use, I recommend topical products with THC and CBD. When applied topically, the THC will not cause psychoactive effects. The following topicals have THC and CBD:
I hope this helps!