To fix the problem you have to know and understand what causes it and treat the cause, not just try to treat or hide the symptoms only. Modern medicine only treats the symptoms.
Psoriasis is caused by the skin being deficient in vitamins A and D.
It is also triggered by stress and the body being deficient in vitamin B.
If the body is in an acid condition and dehydrated, you cannot properly absorb vital nutrients, so you need to drink more good quality alkaline water to rehydrate the skin from within.
Psoriasis causes the skin to be dry, itchy, scaly and quite unsightly. It can be quite debilitating as it becomes so painful and embarrassing.
The skin reproduces far faster and the dead skin does not fall away, and the skin underneath I comes through quite damaged, and still deficient in the nourishing nutrients it needs to be healthy.
The best cure of all is to keep the skin moist and nourished from both inside and out.
The best thing you can do is cover the area with a top quality first cold pressed olive oil, or almond oil, or coconut oil, or margarine every half hour for at least a month until the scale falls away and the new healthy skin comes through.
You can also apply a vitamin A nappy rash cream a couple of times a day.
To prevent it coming back again increase the amount of natural cold pressed oils to diet as well as omega 3 and omega 6 fatty acids from flaxseed oil and sesame oil as well as alkalizing the body and drinking more alkaline water so the body does not become dehydrated, as that caused the skin to become dehydrated too.
You also need to eat more vitamin B foods like mushrooms, fermented foods, promite, vegemite and marmite are also very high in vitamin B.
An important point to remember, anything you put onto the skin goes into the body, so you should never ever put anything on your body that you would not put in your mouth and eat. Most prescription creams doctors prescribe come with warnings saying POISON - NOT TO BE TAKEN.
These things should always be avoided as the skin absorbs into the blood stream everything you put onto it.
The best soaps to use are natural vegetable based ones that are not full of caustic soda. It is also not good to wash with soap too often, as that dries the skin out too.
Try to avoid chlorinated water if possible, until skin has repaired and healed.