Which autoimmune disease primarily affects joints and surrounding muscle tissue?

Rheumatoid arthritis is characterized as an autoimmune disease that primarily impacts the joints and the surrounding muscle tissue. In this condition, the immune system mistakenly attacks the synovium—the lining of the membranes that surround the joints—leading to inflammation, pain, swelling, and eventually potential joint damage. This inflammatory response can also affect adjacent muscle tissue, contributing to decreased function and mobility in affected individuals.

The other conditions listed have different primary targets or symptoms. Osteoporosis primarily involves the weakening of bones, making them more susceptible to fractures rather than directly affecting joints or muscle tissue. Dermatopolymyositis is related to inflammation of the skin and muscle tissues but is not primarily a joint disease. Myasthenia gravis affects the communication between nerves and muscles, leading to weakness but does not primarily involve the joints. Therefore, rheumatoid arthritis is correctly identified as the autoimmune disease that chiefly affects joints and surrounding muscle tissue.