**Core Concept**
Mifepristone is a medication used for medical abortion, which involves blocking the action of a hormone essential for maintaining pregnancy. This hormone is produced by the placenta and plays a crucial role in fetal development.

**Why the Correct Answer is Right**
Mifepristone acts by blocking the progesterone receptors in the uterus. Progesterone is a hormone that prepares and maintains the uterine lining for implantation of the fertilized egg. By blocking progesterone receptors, mifepristone prevents the growth and maintenance of the uterine lining, leading to the termination of pregnancy. This action is mediated through the progesterone receptor, which is a nuclear receptor that regulates gene expression in response to progesterone binding.

**Why Each Wrong Option is Incorrect**
* **Option A:** Mifepristone does not act on the estrogen receptors, which are involved in the regulation of the menstrual cycle and the growth of the endometrium.
* **Option B:** Mifepristone does not act on the prostaglandin receptors, which are involved in the process of labor and delivery.
* **Option C:** Mifepristone does not act on the oxytocin receptors, which are involved in the stimulation of uterine contractions.

**Clinical Pearl / High-Yield Fact**
Mifepristone is a potent glucocorticoid receptor antagonist, which means it can also be used to treat Cushing's syndrome, a condition caused by excess cortisol production.

**Correct Answer: A. Mifepristone acts on progesterone receptors.**