Hello everyone I wanted to ask about the appear...

How to diagnose the cause  the cause of skin rash?
There are many causes and those include:
Allergy,
Dermatological disease
Side effects of drugs
Systemic illness
Local infection
 
You need to do the tests:
CBC, ESR, CRP, RF, AST, ALT, creatinin, urea
 
How to manage skin lesion due to allergy?
You need to use those medications;
If there is  wide space of skin rash in this case you need to use dexamethasone injection and also you need to use topical hydrocortisone 1%
I also recommend to use antihistamine medications to ease symptoms.
If rash last more than one day in this case you need to consult dermatologist to help you and to follow up treatment.