Which of the following is classified as an organ-specific autoimmune disease?

Addison's disease is classified as an organ-specific autoimmune disease because it primarily affects the adrenal glands, which are responsible for producing hormones such as cortisol and aldosterone. In Addison's disease, the body's immune system mistakenly attacks and damages the adrenal cortex, leading to insufficient hormone production. This direct targeting of a specific organ distinguishes it as an organ-specific autoimmune condition.

In contrast, the other conditions mentioned involve systemic effects or multiple organs and tissues. For instance, rheumatoid arthritis affects the joints but also has systemic manifestations, progressive systemic sclerosis (scleroderma) involves widespread effects on the skin and internal organs, and myasthenia gravis primarily affects neuromuscular transmission but can relate to other systems as well. Thus, Addison's disease stands out as a clear example of an autoimmune disease that specifically targets a single organ.