What could be the words for a scale/mark on a certain part of the skin which is small and not spreading(on any other part of the body) like the word ‘limited’? Example: “Your psoriasis mark, it’s ‘confined’(add your suggested word here)”
"Contained" is the expression normally used when a virus or infection has been prevented from spreading among the population (ie from person to person).
Your suggestion of "confined to..." seems perfectly accurate for something visible, such as a rash, which is only one particular area, but I believe the medical term would be localised. Localized means the rash occurs on one small part of the body.