**Core Concept**
The patient's presentation of Raynaud's phenomenon, arthralgia, acrosclerosis, and mild myositis, along with positivity for U1-RNP antibody, suggests a systemic autoimmune disorder. U1-RNP antibodies are a hallmark of mixed connective tissue disease (MCTD), a condition characterized by overlapping features of lupus, scleroderma, and polymyositis.

**Why the Correct Answer is Right**
The U1-RNP antibody is directed against a complex of small nuclear ribonucleoproteins (snRNPs) that are involved in RNA splicing. In MCTD, the immune system mistakenly targets these snRNPs, leading to the activation of various cellular pathways that result in the clinical manifestations of the disease. The presence of U1-RNP antibodies is a key diagnostic criterion for MCTD, which is often seen in young women.

**Why Each Wrong Option is Incorrect**
* **Option A:** While systemic lupus erythematosus (SLE) is an autoimmune disease that can present with Raynaud's phenomenon and myositis, it is not typically associated with U1-RNP antibodies. SLE is more commonly associated with anti-dsDNA and anti-Sm antibodies.
* **Option B:** Scleroderma is a condition characterized by fibrosis of the skin and internal organs, but it is not typically associated with U1-RNP antibodies. Scleroderma is more commonly associated with anti-SCL-70 (anti-topoisomerase I) antibodies.
* **Option D:** Polymyositis is an inflammatory muscle disease that can present with myositis, but it is not typically associated with U1-RNP antibodies. Polymyositis is more commonly associated with anti-Jo-1 antibodies.

**Clinical Pearl / High-Yield Fact**
The presence of U1-RNP antibodies in a young woman with Raynaud's phenomenon, arthralgia, and myositis should prompt consideration of mixed connective tissue disease (MCTD). MCTD is a treatable condition that requires a multidisciplinary approach to management.

**Correct Answer: C. Mixed Connective Tissue Disease (MCTD)**