## **Core Concept**
Leflunomide is an immunosuppressive disease-modifying antirheumatic drug (DMARD) used in the treatment of rheumatoid arthritis. It works by inhibiting dihydroorotate dehydrogenase, an enzyme necessary for the production of pyrimidines, which are required for the proliferation of T and B lymphocytes. A loading dose is often used to rapidly achieve therapeutic levels of a drug.

## **Why the Correct Answer is Right**
The correct loading dose of leflunomide in rheumatoid arthritis is **100 mg**. This high initial dose helps to quickly achieve steady-state plasma concentrations, allowing for a faster onset of action. After the loading dose, the maintenance dose is significantly lower, typically **20 mg/day**, to maintain therapeutic levels while minimizing side effects.

## **Why Each Wrong Option is Incorrect**
- **Option A:** 20 mg - This is the typical maintenance dose, not the loading dose.
- **Option B:** 10 mg - This is below the standard loading dose and would delay achieving therapeutic levels.
- **Option D:** 40 mg - Although higher doses might be considered in some cases, **100 mg** is the standard loading dose.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that leflunomide requires a loading dose to rapidly achieve therapeutic efficacy in rheumatoid arthritis. The standard regimen starts with **100 mg/day for 3 days** as a loading dose, followed by **20 mg/day** as a maintenance dose.

## **Correct Answer:** . **100 mg**