**Core Concept**
Rheumatoid arthritis (RA) and osteoarthritis (OA) are two distinct types of arthritis with different underlying pathophysiologies. RA is an autoimmune disease characterized by inflammation of the synovium, leading to joint damage and deformity. In contrast, OA is a degenerative joint disease primarily caused by wear and tear of the articular cartilage.

**Why the Correct Answer is Right**
The primary goal of treatment in RA is to suppress the autoimmune response and reduce inflammation. Non-steroidal anti-inflammatory drugs (NSAIDs) and disease-modifying antirheumatic drugs (DMARDs) are commonly used to manage RA symptoms. One class of DMARDs that is particularly effective for RA is methotrexate, a folate antagonist that inhibits the synthesis of pyrimidines and thus suppresses lymphocyte proliferation.

*Option A: Methotrexate* is a DMARD that is effective for RA due to its ability to inhibit lymphocyte proliferation and reduce inflammation. Methotrexate is not suitable for OA because it does not address the degenerative changes in the joint and may even exacerbate the condition by inhibiting cartilage repair.

**Why Each Wrong Option is Incorrect**
* **Option B:** Hydroxychloroquine is a DMARD that is often used in combination with methotrexate for RA. While it is not the best answer, hydroxychloroquine is still a valid treatment option for RA.

* **Option C:** Celecoxib is a COX-2 inhibitor that is effective for OA by reducing inflammation without inhibiting cartilage repair. Celecoxib is not typically used for RA.

* **Option D:** Paracetamol is a weak analgesic and antipyretic that is often used for OA pain management. Paracetamol is not effective for RA and is not considered a first-line treatment.

**Clinical Pearl / High-Yield Fact**
When choosing a treatment for arthritis, it's essential to distinguish between RA and OA based on clinical presentation and diagnostic tests. RA often presents with symmetrical joint involvement, morning stiffness, and systemic symptoms, whereas OA typically presents with localized joint pain and stiffness.

**Correct Answer: A. Methotrexate**