Hello, I read your message carefully and understood your medical issue. I need to see pictures of your arm in order to give a proper opinion about what it can be. Eczema is also called atopic dermatitis. It is always itchy and arms are not a common site. The rash usually starts on the face, back of the knees, hands, and feet. The skin becomes dry, thickened or scaly. The cause is unknown but it has a family predisposes in people who have a family history of asthma or allergies. Psoriasis is a skin disorder which causes the skin cells to multiply very fast. It is presented in the skin as red plaques covered with white scales. The usual sites are knees, scalp, and elbows than it may spread in other areas too. The skin lesions may be itchy and painful. These two conditions are not very usual to happen. You may have just a contact dermatitis due to the allergy from some chemical found in the environment (for example the detergent you used to wash your t-shirt) or any type of skin infection. I think you must consult a dermatologist for further evaluation. If you can please send me photos of you arm and I will be able to say my final opinion about the nature of your skin lesions. Thank you.