A blotchy rash on the elbows and knees is a common symptom of plaque psoriasis, a skin condition that causes skin cells to multiply up to 10 times faster than normal, according to WebMD. WebMD explains that plaque psoriasis causes red plaques on the skin that are often covered with loose scales. The plaques tend to itch and bleed and are commonly found on the elbows, knees and scalp, as noted by WebMD.
The American Academy of Dermatology (ADD) explains that plaque psoriasis patches range from single plaques to several plaques joined together over a single area of skin. The AAD warns that scratching the patches can cause thickened skin. Guttate psoriasis is another form of the skin condition that causes small red spots on the arms, trunk and legs and sometimes on the scalp, face and ear, as noted by the AAD. Guttate psoriasis spots tend to appear after a bout of illness, according to the AAD.
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. Additional symptoms of atopic dermatitis include oozing blisters and dry skin all over the body, according to MedlinePlus.