**Core Concept**
Methotrexate is a disease-modifying antirheumatic drug (DMARD) used in the treatment of rheumatoid arthritis (RA). It works by inhibiting dihydrofolate reductase (DHFR), an enzyme involved in the synthesis of tetrahydrofolate, a crucial cofactor for DNA synthesis and repair.

**Why the Correct Answer is Right**
Methotrexate is initiated at a low dose, typically 7.5-10 mg/week, orally or subcutaneously. The dose is gradually increased every 1-2 weeks, based on clinical response and tolerance, to a maximum dose of 20-25 mg/week. This stepwise approach helps minimize side effects, such as gastrointestinal toxicity and hepatotoxicity. The optimal dose of methotrexate for RA treatment is usually achieved when the patient experiences a significant reduction in disease activity, as measured by the American College of Rheumatology (ACR) response criteria.

**Why Each Wrong Option is Incorrect**
**Option A:** 25 mg/day - This dose is higher than the recommended maximum dose for RA treatment and may increase the risk of adverse effects.

**Option B:** 1 mg/week - This dose is lower than the typical starting dose for RA treatment and may not provide adequate therapeutic effects.

**Option C:** 50 mg/month - This dosing schedule is not standard for methotrexate in RA treatment and may lead to inconsistent drug levels and reduced efficacy.

**Clinical Pearl / High-Yield Fact**
When initiating methotrexate therapy, it is essential to monitor liver function tests (LFTs) and complete blood counts (CBCs) regularly, as methotrexate can cause hepatotoxicity and myelosuppression.

**Correct Answer: C. 15 mg/week**