Hi Welcome to HEALTHCARE MAGIC..
I have gone through your query and can understand your concern...
Yes
psoriasis can cause itchy rashes on back, stomach and arms. But we have to ruleout other causes of rashes before this. Other causes of rash include
1.Rashes due to contact with allergic and poisonous agent found in herbs like
poison oak or ivy, or present in the insect’s sting or animal’s bite.
2. Rashes due to friction between body parts & clothes. This type is common in cyclist,joggers,etc. Always take a bath after such physical activity. Don't use tight clothes.
3.Psoriasis,
impetigo and eczema are few of the diseases that are known to produce rashes or skin eruption.
4. Rashes can be due to viral fever
5.
fungal infection can cause itchy rashes
Normally rashes due to viral infection or allergy will heal itself with time.
If problem persist, You should
consult a dermatologist & give him proper history of psoriasis.
Hope this helps