## Core Concept
Mifepristone, also known as RU486, is a **progesterone receptor antagonist** used in medical abortion and emergency contraception. It works by blocking the action of progesterone, which is essential for maintaining pregnancy. In the context of emergency contraception, mifepristone's role is to prevent ovulation or implantation of a fertilized egg.

## Why the Correct Answer is Right
The correct dose of mifepristone for emergency contraception is **1.5 mg**. This dose has been studied and found effective in preventing pregnancy when used as emergency contraception. Mifepristone at this dose acts by delaying or inhibiting ovulation, and possibly affecting the endometrium, thereby preventing pregnancy.

## Why Each Wrong Option is Incorrect
- **Option A:** 10 mg - This dose is actually used for medical abortion, not emergency contraception. The higher dose is required for terminating an existing pregnancy.
- **Option B:** 5 mg - While lower doses of mifepristone have been explored for various applications, 5 mg is not the standard dose for emergency contraception.
- **Option D:** 20 mg - This is also higher than the required dose for emergency contraception and is not typically used for this purpose.

## Clinical Pearl / High-Yield Fact
A key point to remember is that mifepristone is most effective as emergency contraception when taken as soon as possible after unprotected sex, ideally within 72 hours. The efficacy decreases but is still present up to 120 hours after unprotected sex. Mifepristone offers a non-hormonal alternative to traditional hormonal emergency contraceptives like the levonorgestrel regimen.

## Correct Answer Line
**Correct Answer: C. 1.5 mg.**