**Core Concept**
Still's disease, also known as juvenile rheumatoid arthritis (JRA), is a type of autoimmune disorder characterized by chronic inflammation and joint pain in children. It is a systemic disease that affects multiple organ systems, including the skin, joints, and other tissues.

**Why the Correct Answer is Right**
The characteristic feature of Still's disease is the presence of a distinctive rash, known as the Still's rash or salmon-colored rash. This rash typically appears on the trunk and limbs and is often accompanied by fever, joint pain, and swelling. The rash is thought to be a result of the systemic inflammatory response and the release of cytokines, such as interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha), which play a key role in the pathogenesis of the disease. The rash is a critical diagnostic feature of Still's disease, and its presence helps to differentiate it from other forms of arthritis.

**Why Each Wrong Option is Incorrect**
**Option A:** Prominent kidney involvement is not a characteristic feature of Still's disease. While kidney problems can occur in some cases of JRA, they are not a hallmark of the disease.
**Option C:** A positive Rheumatoid factor is not typically associated with Still's disease. Rheumatoid factor is more commonly seen in adult rheumatoid arthritis and is not a diagnostic feature of JRA.
**Option D:** Neutropenia is not a characteristic feature of Still's disease. In fact, many patients with JRA may have an elevated white blood cell count due to the systemic inflammatory response.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the Still's rash is a critical diagnostic feature of juvenile rheumatoid arthritis and can help differentiate it from other forms of arthritis. The rash is often accompanied by fever, joint pain, and swelling, and its presence should prompt further investigation into the diagnosis of JRA.

**✓ Correct Answer: B. Rashes**