**Core Concept**
Rheumatoid arthritis (RA) and Systemic Lupus Erythematosus (SLE) are both autoimmune disorders that can present with polyarthritis, but they have distinct clinical and serological characteristics. Differentiating between these two conditions is crucial for proper diagnosis and treatment.

**Why the Correct Answer is Right**
The presence of rheumatoid factor (RF) and anti-nuclear antibodies (ANA) can be seen in both RA and SLE, making it challenging to differentiate between the two based on these markers alone. However, the anti-cyclic citrullinated peptide (anti-CCP) antibody is more specific for RA, whereas the anti-Sm and anti-RNP antibodies are more specific for SLE. **Option D**, anti-CCP antibody, would be the correct choice as it can help differentiate RA from SLE.

**Why Each Wrong Option is Incorrect**
**Option A:** RF is present in both RA and SLE, making it a non-specific marker for both conditions.
**Option B:** ANA is also present in both RA and SLE, and is not specific enough to differentiate between the two.
**Option C:** This option is incomplete, but assuming it's another antibody, it's likely not specific enough to differentiate between RA and SLE.

**Clinical Pearl / High-Yield Fact**
Remember the "RA-SLE" mnemonic: RA is associated with RF and anti-CCP, while SLE is associated with ANA, anti-Sm, and anti-RNP. This can help you differentiate between the two conditions.

**Correct Answer:** D. anti-CCP antibody.