brimstone - an old name for sulfur. native sulfur, native sulphur. atomic number 16, sulfur, sulphur, S - an abundant tasteless odorless multivalent nonmetallic element; best known in yellow crystals; occurs in many sulphide and sulphate minerals and even in native form (especially in volcanic regions)
These patches of red, and sometimes irritated or itchy, skin can appear anywhere on the body, but they are most commonly found on the arms, neck, and face. These blotchy patches can be caused by a variety of factors, such as weather, genetics, allergies, or environmental conditions.
According to MedlinePlus, a blotchy rash on the elbows and inside the knees is a symptom of atopic dermatitis, a skin reaction that causes swelling and redness. MedlinePlus explains that atopic dermatitis is common in individuals who have seasonal allergies, but the rash itself is not caused by allergies.