**Core Concept**
Mifepristone is a synthetic steroid that acts as a potent antagonist of the progesterone receptor, thereby inhibiting progesterone's effects on the uterus. This unique mechanism of action makes mifepristone a valuable medication in the management of certain gynecological conditions.

**Why the Correct Answer is Right**
Mifepristone is primarily used in the medical management of early pregnancy termination due to its ability to block the action of progesterone, a hormone essential for maintaining pregnancy. By inhibiting progesterone receptors, mifepristone prevents the implantation of the fertilized egg in the uterine lining, effectively terminating the pregnancy. This mechanism of action also makes mifepristone useful in the treatment of conditions such as Cushing's syndrome, where excessive levels of cortisol (a hormone similar to progesterone) can cause debilitating symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because mifepristone is not typically used as a treatment for endometriosis, although it may be used off-label in some cases.
**Option B:** This option is incorrect because while mifepristone can be used to treat Cushing's syndrome, it is not the most common indication for this medication.
**Option C:** This option is incorrect because mifepristone is not typically used to treat polycystic ovary syndrome (PCOS), although it may be used off-label in some cases.

**Clinical Pearl / High-Yield Fact**
Mifepristone's mechanism of action as a progesterone receptor antagonist makes it a valuable tool in the management of early pregnancy termination, but it also highlights the importance of understanding the role of progesterone in maintaining pregnancy.

**Correct Answer:** C. Mifepristone is used for the medical management of early pregnancy termination.