**Core Concept**
Selective progesterone receptor modulators (SPRMs) are a class of drugs that act by binding to the progesterone receptor and modulating its activity. They are used in the treatment of conditions such as uterine fibroids, endometriosis, and in the management of hormone receptor-positive breast cancer. SPRMs can have both agonist and antagonist effects on the progesterone receptor, depending on the specific tissue or cell type.

**Why the Correct Answer is Right**
Mifepristone is a well-known SPRM that acts by binding to the progesterone receptor, preventing it from binding to its endogenous ligand, progesterone. This results in a reduction in progesterone-mediated effects, such as uterine contraction and endometrial growth. Mifepristone is used in the medical management of early pregnancy termination and in the treatment of Cushing's syndrome. In the context of breast cancer, mifepristone can act as an antagonist, reducing the growth-promoting effects of progesterone on breast cancer cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Progesterone itself is not a selective progesterone receptor modulator, as it acts as a full agonist on the progesterone receptor. It does not have the ability to modulate its activity in a tissue-specific manner.
**Option B:** This option is blank. Please provide the distractors.
**Option C:** This option is blank. Please provide the distractors.
**Option D:** This option is blank. Please provide the distractors.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that SPRMs like mifepristone can have varying effects on different tissues and cell types, and their use should be carefully considered based on the specific clinical context. Additionally, SPRMs can interact with other hormone therapies, such as tamoxifen, to produce synergistic or antagonistic effects.

**Correct Answer: B. Mifepristone**