**Core Concept**
Anti-progesterone agents like RU-486 (mifepristone) act by competitively inhibiting the progesterone receptor, thereby disrupting the normal progesterone-mediated effects on the endometrium and decidua, which are crucial for maintaining pregnancy.

**Why the Correct Answer is Right**
RU-486, also known as mifepristone, is a potent anti-progesterone agent that works by binding to the progesterone receptor and preventing progesterone from exerting its effects on the endometrium. This disrupts the normal decidualization process, leading to the breakdown of the endometrial lining and termination of pregnancy. Mifepristone's action is based on its high affinity for the progesterone receptor, which is essential for maintaining pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** Mifepristone is not a selective estrogen receptor modulator (SERM) like tamoxifen, which acts on the estrogen receptor rather than the progesterone receptor.
**Option B:** Mifepristone is not a prostaglandin analogue like misoprostol, which is used to induce labor and terminate pregnancy but works through a different mechanism.
**Option C:** Mifepristone is not a gonadotropin-releasing hormone (GnRH) agonist like leuprolide, which inhibits gonadotropin secretion but does not directly affect progesterone levels or action.

**Clinical Pearl / High-Yield Fact**
It is essential to note that mifepristone is used in combination with a prostaglandin analogue like misoprostol for medical abortion, as the anti-progesterone effect of mifepristone is incomplete without the induction of uterine contractions by the prostaglandin analogue.

**Correct Answer:** C. Mifepristone