**Core Concept**
The question tests the student's knowledge of the serological markers for autoimmune diseases, specifically scleroderma. Scleroderma is a chronic autoimmune disorder characterized by the deposition of collagen, leading to skin and internal organ fibrosis. The screening test for scleroderma involves detecting the presence of autoantibodies.

**Why the Correct Answer is Right**
The correct answer is **A. Anti-nuclear antibody (ANA)**. ANA is a broad-spectrum autoantibody that targets various components of the cell nucleus, including histones, DNA, and RNA. In scleroderma, ANA is present in approximately 90-100% of patients, making it a sensitive screening test for the disease. The presence of ANA suggests an autoimmune process, but it does not differentiate between various autoimmune diseases. Other autoantibodies, such as anti-RNP and anti-Scl-70 (topoisomerase I), are more specific for scleroderma but are not as sensitive as ANA.

**Why Each Wrong Option is Incorrect**
**Option B:** Anti-U1-Ribonucleoprotein (RNP) antibody is more specific for mixed connective tissue disease (MCTD), which is a distinct autoimmune disorder that shares features with scleroderma, lupus, and rheumatoid arthritis.
**Option C:** Anti-LKM (Liver-Kidney Microsomal) antibody is associated with autoimmune hepatitis and drug-induced liver injury, not scleroderma.
**Option D:** Anti-topoisomerase I (Scl-70) antibody is a specific marker for scleroderma, particularly for the diffuse form of the disease, but it is not used as a screening test due to its lower sensitivity compared to ANA.

**Clinical Pearl / High-Yield Fact**
Remember that a positive ANA result does not necessarily diagnose scleroderma, as it can be present in various autoimmune diseases. A high-titer ANA (>1:160) with a homogeneous or speckled pattern is more suggestive of scleroderma.