**Core Concept**
Rheumatoid arthritis is a chronic inflammatory disorder that affects the joints, and patients with RA often require long-term treatment with disease-modifying antirheumatic drugs (DMARDs). One class of DMARDs is the sulfasalazine class, which includes sulfasalazine and its derivatives like azathioprine, however the most relevant one here is a different derivative called Leflunomide.

**Why the Correct Answer is Right**
Leflunomide is a pyrimidine synthesis inhibitor that works by inhibiting the enzyme dihydroorotate dehydrogenase (DHODH), which is necessary for the synthesis of pyrimidines and thus DNA. However, one of its known side effects is an increased risk of optic neuritis and subsequent vision loss in patients with RA. This is thought to occur due to the inhibition of pyrimidine synthesis in the retinal cells, leading to inflammation and damage to the optic nerve.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while sulfasalazine can cause various side effects, it is not typically associated with vision loss. Sulfasalazine's mechanism of action involves inhibiting the enzyme xanthine oxidase, which is involved in purine metabolism.
**Option B:** This option is incorrect because azathioprine is a different DMARD that works by inhibiting the enzyme IMP dehydrogenase, which is involved in purine synthesis. While azathioprine can cause various side effects, it is not typically associated with vision loss.
**Option C:** This option is incorrect because methotrexate is a different DMARD that works by inhibiting the enzyme dihydrofolate reductase, which is involved in folate metabolism. While methotrexate can cause various side effects, it is not typically associated with vision loss.

**Clinical Pearl / High-Yield Fact**
It's essential to monitor patients on leflunomide for signs of optic neuritis, such as blurred vision, pain on eye movement, or loss of color vision, and to discontinue the medication if these symptoms occur.

**Correct Answer:** C.