## **Core Concept**
Mifepristone, also known as RU486, is a medication used for medical abortion. It works by blocking the **progesterone receptors**, which are essential for maintaining pregnancy. By inhibiting these receptors, mifepristone causes the uterine lining to shed, and the pregnancy cannot continue.

## **Why the Correct Answer is Right**
The standard dose of mifepristone for medical abortion is **200 mg**, which can be taken orally as a single dose or split into two doses of 100 mg, 12 hours apart. This dosage has been shown to be effective in terminating early pregnancies when followed by another medication, usually misoprostol, which causes uterine contractions to expel the pregnancy.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Lower doses might not be as effective for terminating pregnancy, potentially leading to incomplete abortion or continued pregnancy.
- **Option B:** While some studies explore different dosing regimens, 600 mg is higher than the standard recommended dose and may increase the risk of side effects without providing additional benefits.
- **Option D:** A dose of 100 mg might be used in some protocols but is not the standard dose when considering efficacy and protocols approved by major health organizations.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that mifepristone is most effective for medical abortion when used in conjunction with misoprostol, and its efficacy is highly dependent on the gestational age of the pregnancy, being most effective up to 7 weeks of gestation.

## **Correct Answer:** B. 200 mg.