**Core Concept**
Progesterone plays a crucial role in emergency contraception by inhibiting or delaying ovulation and modifying the endometrium to prevent implantation of a fertilized egg. The progesterone used in emergency contraception should have a high efficacy, rapid onset of action, and minimal side effects.

**Why the Correct Answer is Right**
The progesterone of choice in emergency contraception is Ulipristal acetate (EA). It is a selective progesterone receptor modulator (SPRM) that works by binding to the progesterone receptor and altering its function. This leads to a delay in ovulation and a change in the endometrium, making it less receptive to implantation. Ulipristal acetate has a high efficacy rate, with a Pearl Index of approximately 1.5 per 100 women. It also has a rapid onset of action, with a peak effect within 24 hours of administration.

**Why Each Wrong Option is Incorrect**
**Option A:** Medroxyprogesterone acetate (MPA) is a long-acting progestin used in contraceptive injections and implants, but it is not the preferred choice for emergency contraception due to its slower onset of action and lower efficacy rate.

**Option B:** Levonorgestrel (LNG) is a progestin used in emergency contraception, but it is less effective than ulipristal acetate (EA) and has a lower efficacy rate. LNG is also associated with more side effects, such as nausea and vomiting.

**Option C:** Norethisterone is an older progestin that has been used in various contraceptive formulations, but it is not the preferred choice for emergency contraception due to its lower efficacy rate and potential side effects.

**Clinical Pearl / High-Yield Fact**
Ulipristal acetate (EA) is a more effective and safer option for emergency contraception compared to levonorgestrel (LNG) and other progestins. It is also effective when taken up to 120 hours (5 days) after unprotected sex, making it a valuable option for women who may have delayed seeking medical attention.

**Correct Answer: . Ulipristal acetate (EA)**