**Core Concept**
Mifepristone, also known as RU-486, is an antiprogestin medication primarily used for medical abortion in the early weeks of pregnancy. It works by blocking the action of progesterone, a hormone essential for maintaining pregnancy.

**Why the Correct Answer is Right**
Mifepristone specifically targets the progesterone receptor, a cytoplasmic receptor that plays a crucial role in the maintenance of pregnancy. By blocking the progesterone receptor, mifepristone prevents the implantation of the fertilized egg and causes the uterine lining to shed, leading to abortion. This mechanism of action makes it an effective medication for inducing abortion in the early weeks of pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** Although mifepristone is used for inducing abortion in the early week of pregnancy, option A does not specify the correct timing. Medical abortion typically involves mifepristone in combination with another medication, such as misoprostol, which is used in the second week of pregnancy.

**Option B:** Mifepristone is not used along with a contraceptive pill. Its primary use is for medical abortion, not for preventing pregnancy.

**Option C:** Mifepristone acts on the progesterone receptor, which is a nuclear receptor, not a cytoplasmic receptor.

**Option D:** Mifepristone does not prevent ectopic implantation; it prevents implantation in the uterus.

**Clinical Pearl / High-Yield Fact**
Mifepristone is a potent antiprogestin that blocks the action of progesterone, a hormone essential for maintaining pregnancy. This mechanism of action makes it an effective medication for medical abortion in the early weeks of pregnancy.

**Correct Answer:**
✓ Correct Answer: A. Used for inducing aboion in early week of pregnancy