Which of the following progesterone is used in emergency contraception?
**Core Concept**
Progesterone plays a crucial role in emergency contraception by altering the endometrium and preventing implantation of the fertilized ovum. This mechanism is distinct from the hormonal regulation of the menstrual cycle.
**Why the Correct Answer is Right**
The correct answer is a progesterone derivative that is used in emergency contraception. Ulipristal acetate (C) is a selective progesterone receptor modulator (SPRM) that acts by binding to the progesterone receptor, thereby preventing the endometrial transformation necessary for implantation. This mechanism is distinct from the hormonal regulation of the menstrual cycle.
**Why Each Wrong Option is Incorrect**
* **Option A:** Levonorgestrel is a progesterone derivative used in oral contraceptives, but it is not the correct answer for emergency contraception in this context.
* **Option B:** Mifepristone is an antiprogestin used in medical abortion, not emergency contraception.
* **Option D:** Norgestrel is a progesterone derivative used in oral contraceptives and IUDs, but it is not the correct answer for emergency contraception in this context.
**Clinical Pearl / High-Yield Fact**
Ulipristal acetate is a more effective option for emergency contraception than levonorgestrel, especially when taken within 72 hours of unprotected sex. However, it should not be used as a regular contraceptive method due to its high cost and potential side effects.
**Correct Answer:** C. Ulipristal acetate.