**Question:** Common antibody in Polymyositis and Dermatomyositis includes

A. Joining Region of Immunoglobulin M (Jo-1)
B. Mitochondrial Antigen (Mi-2)
C. RNA Polymerase III Antibody
D. Calcium Channel Antibody

**Core Concept:** Polymyositis and Dermatomyositis are autoimmune diseases characterized by inflammation of the skeletal muscles and skin respectively. These diseases are often associated with specific autoantibodies.

**Why the Correct Answer is Right:** The correct answer, C. RNA Polymerase III Antibody, is found in a significant proportion of patients with dermatomyositis. RNA Polymerase III is a key enzyme involved in RNA synthesis, and its inhibition leads to muscle cell dysfunction and inflammation.

**Why Each Wrong Option is Incorrect:**
A. Joining Region of Immunoglobulin M (Jo-1) is a specific autoantibody found in approximately 10% of patients with dermatomyositis and polymyositis. However, it is not specific to dermatomyositis and may also be present in other autoimmune diseases.
B. Mitochondrial Antigen (Mi-2) is a specific autoantibody found in around 20% of patients with dermatomyositis, but it is also found in other autoimmune diseases.
D. Calcium Channel Antibody is not specific to dermatomyositis and is more commonly associated with other autoimmune diseases.

**Clinical Pearl:** Understanding the specific autoantibodies associated with dermatomyositis and polymyositis can aid in diagnosing these conditions and distinguishing them from other autoimmune diseases. A positive test for RNA Polymerase III Antibody in a patient presenting with dermatomyositis symptoms can help confirm the diagnosis.

**Correct Answer:** C. RNA Polymerase III Antibody

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In summary, the correct answer is C. RNA Polymerase III Antibody because it is specific to dermatomyositis and helps in diagnosing the disease. However, it is essential to consider other autoimmune diseases as well when assessing a patient, as some antibodies may overlap between conditions.