**Core Concept:** Leflunomide is an immunosuppressive drug used in the treatment of rheumatoid arthritis (RA). It works by inhibiting the enzyme dihydroorotate dehydrogenase (DHODH), which leads to reduced pyrimidine synthesis and ultimately suppresses T-cell proliferation. Loading doses are used to achieve therapeutic drug levels more quickly.

**Why the Correct Answer is Right:** Leflunomide is given as a loading dose to achieve a rapid increase in drug levels to suppress immune system activity in RA. In this case, the correct answer is **A**. A loading dose of 160 mg is administered orally on the first day, followed by a maintenance dose of 10 mg daily.

**Why Each Wrong Option is Incorrect:**

A. **Option A (120 mg):** This is incorrect because it is higher than the recommended loading dose of 160 mg on the first day.
B. **Option B (5 mg):** This is incorrect because it is too low a dose to effectively suppress immune system activity in RA.
C. **Option D (20 mg):** This is incorrect because it is higher than the recommended maintenance dose of 10 mg daily, leading to potential side effects and toxicity.

**Why Leflunomide is Used:** Leflunomide is used in RA due to its efficacy in reducing disease activity and improving functional status in patients. By administering a higher initial dose (loading dose), the drug reaches therapeutic levels more rapidly, ensuring better treatment outcomes.

**Clinical Pearl:** In RA patients, achieving early disease control is crucial for minimizing joint damage and improving long-term outcomes. A proper dosing regimen (loading dose followed by maintenance dose) ensures that leflunomide reaches therapeutic levels, suppressing immune system activity effectively.