Which autoimmune disease primarily affects the body's joints?

Rheumatoid arthritis is the correct choice as it is an autoimmune disease specifically characterized by chronic inflammation of the joints. This condition occurs when the immune system mistakenly attacks the synovium, which is the lining of the membranes that surround the joints. This attack leads to painful swelling, and over time, it can result in joint damage and decreased function.

In contrast, while multiple sclerosis affects the nervous system, lupus can affect multiple systems including skin, kidneys, and joints but is not primarily a joint condition, and psoriasis primarily affects the skin, although a form of arthritis associated with psoriasis can occur. Therefore, the defining feature of rheumatoid arthritis is its significant impact on joint health, making it distinct among autoimmune diseases.