**Core Concept**
Primary idiopathic polymyositis is a chronic inflammatory muscle disease characterized by progressive muscle weakness and wasting, primarily affecting the proximal muscles. It is a type of autoimmune disorder, where the immune system mistakenly attacks the muscle fibers, leading to muscle damage and weakness.

**Why the Correct Answer is Right**
The correct answer involves understanding the pathophysiology of polymyositis. In primary idiopathic polymyositis, the immune system produces autoantibodies against the muscle fibers, leading to inflammation and muscle damage. This process is mediated by various immune cells, including T cells and macrophages, which release pro-inflammatory cytokines and enzymes that damage the muscle fibers. The hallmark of polymyositis is the presence of muscle fiber necrosis and regeneration, which is a result of the immune system's attack on the muscle tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a characteristic of polymyositis. Polymyositis is primarily a muscle disease, and it does not typically involve the nervous system.

**Option B:** This option is incorrect because it is not a hallmark of polymyositis. While polymyositis can involve the skin and other organs in some cases, it is primarily a muscle disease.

**Option C:** This option is incorrect because it is not a specific characteristic of polymyositis. While polymyositis can cause muscle weakness and wasting, it does not typically cause joint pain or swelling.

**Clinical Pearl / High-Yield Fact**
One key feature of polymyositis is the presence of muscle fiber necrosis and regeneration, which can be seen on muscle biopsy. This is a classic finding that helps differentiate polymyositis from other muscle diseases.

**Correct Answer:** D