**Core Concept**
Emergency contraception refers to the use of hormonal methods to prevent pregnancy after unprotected sex or contraceptive failure. Various agents can be used to prevent ovulation, fertilization, or implantation. The choice of agent depends on the timing and method of contraception.

**Why the Correct Answer is Right**
Mifepristone (RU486) is a progesterone receptor antagonist that can be used for emergency contraception. It acts by inhibiting ovulation, making the uterine lining inhospitable for implantation, and possibly altering the cervical mucus to prevent sperm penetration. Mifepristone's mechanism is distinct from other emergency contraceptives, such as levonorgestrel, ulipristal acetate, and the Yuzpe regimen (estrogen and progesterone combination), which primarily work by inhibiting ovulation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Mifepristone is indeed used for emergency contraception and is the correct answer.
* **Option B:** Ulipristal acetate is a selective progesterone receptor modulator that can be used for emergency contraception and functions similarly to mifepristone.
* **Option C:** Levonorgestrel is a progestin that can be used for emergency contraception and primarily works by inhibiting ovulation.
* **Option D:** Estrogen and progesterone combination (Yuzpe regimen) can be used for emergency contraception and primarily works by inhibiting ovulation.

**Clinical Pearl / High-Yield Fact**
When prescribing emergency contraception, it's essential to note that the effectiveness decreases with increasing time from unprotected sex. Mifepristone is most effective when taken within 72 hours, while levonorgestrel and ulipristal acetate are most effective when taken within 120 hours.

**Correct Answer:** A.