**Core Concept**
Still's disease, also known as juvenile rheumatoid arthritis, is a rare autoimmune disorder characterized by inflammation of the joints, fever, and other systemic symptoms. It is a classic example of a systemic autoimmune disease that affects children and young adults.

**Why the Correct Answer is Right**
The characteristic feature of Still's disease is the presence of a characteristic salmon-colored rash or "Still's rash," which is a distinctive sign of the disease. This rash typically appears on the trunk and extremities and is accompanied by fever, joint pain, and swelling. The pathophysiology of Still's disease involves an abnormal immune response, with activation of T cells and production of various cytokines.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Still's disease is not primarily characterized by joint deformity, which is more typical of other forms of arthritis such as rheumatoid arthritis.

**Option B:** This option is incorrect because Still's disease is not primarily characterized by skin lesions, which are more typical of conditions such as lupus erythematosus.

**Option C:** This option is incorrect because Still's disease is not primarily characterized by muscle weakness, which is more typical of conditions such as myasthenia gravis.

**Clinical Pearl / High-Yield Fact**
A key feature of Still's disease is the presence of an evanescent rash that fades quickly on pressure, which is a classic clinical correlation to remember.

**Correct Answer: D. Still's rash.**