## **Core Concept**
Mifepristone, also known as RU-486, is a **synthetic steroid** and a **progestin receptor antagonist**. It is primarily used for **medical abortion** but also has applications in **emergency contraception** due to its ability to inhibit or delay ovulation.

## **Why the Correct Answer is Right**
The correct dose of mifepristone for post-coital contraception is **10 mg**. This dose has been established as effective in preventing pregnancy after unprotected sex. Mifepristone works by blocking the action of **progesterone**, which is necessary for ovulation and for the maintenance of early pregnancy. By taking mifepristone, the hormonal environment is altered, preventing ovulation or making the uterine lining inhospitable for implantation of a fertilized egg.

## **Why Each Wrong Option is Incorrect**
- **Option A (1.5 mg):** This dose might be too low to effectively prevent pregnancy in all cases, as the established effective dose for emergency contraception is higher.
- **Option B (20 mg):** Although higher doses of mifepristone are used for medical abortion (typically 200 mg), 20 mg is not the standard dose for post-coital contraception and could unnecessarily increase the risk of side effects.
- **Option D (50 mg):** This dose is much higher than what is required for emergency contraception and would likely increase the risk of adverse effects without providing additional benefit.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that mifepristone is most effective as an emergency contraceptive when taken as soon as possible after unprotected sex, ideally within **72 hours**. Its effectiveness decreases over time but remains a valuable option for preventing unwanted pregnancy.

## **Correct Answer:** . **10 mg**