Which of the following is not true about polymyositis?
**Core Concept**
Polymyositis is a type of inflammatory muscle disease characterized by muscle weakness, inflammation, and degeneration of muscle fibers. It is an idiopathic condition, meaning that the exact cause is unknown, although it is believed to be an autoimmune disorder.
**Why the Correct Answer is Right**
Polymyositis typically affects proximal muscles, such as those in the neck, shoulders, hips, and thighs, leading to difficulty with activities like lifting, climbing stairs, or bending. The inflammation and muscle damage in polymyositis are caused by the activation of immune cells, including T cells and macrophages, which release various cytokines and inflammatory mediators. The exact pathogenesis of polymyositis is complex and multifactorial, involving both genetic and environmental factors.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because polymyositis can indeed cause muscle atrophy, which is a result of prolonged muscle damage and degeneration.
* **Option B:** This option is incorrect because polymyositis is typically associated with elevated levels of muscle enzymes, such as creatine kinase (CK) and aldolase, in the blood, indicating muscle damage.
* **Option D:** This option is incorrect because polymyositis can occur in association with other autoimmune conditions, such as lupus or rheumatoid arthritis.
**Clinical Pearl / High-Yield Fact**
A key clinical feature of polymyositis is the presence of a characteristic electromyogram (EMG) pattern, which shows an abnormal increase in muscle fiber activity, particularly in the proximal muscles. This can help distinguish polymyositis from other muscle disorders.
**Correct Answer: C.**