Generally, a discolored patch on skin isn't cause for concern. Skin discoloration can be triggered by a number of causes, including:

• Atopic dermatitis (eczema). Sometimes a small area of skin becomes irritated, which results in a patch of skin that's lighter in pigment than is the skin around it (pityriasis alba). This is common in children who have eczema. Your son's doctor may recommend applying moisturizer to the affected skin, avoiding drying soaps and protecting the skin from the sun. Your doctor might also prescribe a mild medicated cream, such as hydrocortisone.
• Superficial yeast infection. Another possible cause of a discolored patch on skin is a superficial yeast infection (tinea versicolor), which means that it affects just the top layer of skin. This condition may cause the skin to look either lighter or darker than usual. Treatment may include antifungal creams or shampoos.
• Other skin diseases, including psoriasis and vitiligo, also may cause patchy discoloration of the skin. Most often other lesions typical for these conditions are present on more than one spot of the body. On brown and black skin, psoriasis can leave discolored patches after it heals (postinflammatory hypopigmentation). The skin usually regains its usual color within a few months. Treatment of vitiligo can be challenging. It's important to get a correct diagnosis so that prescription treatments can be considered.

Consult a dermatologist for an evaluation if the discolored patch on your son's arm continues to enlarge or if additional patches appear in other places.