**Core Concept**
Mifepristone is a medication that interacts with specific receptors in the body, leading to its therapeutic effects. It is a synthetic steroid with antiglucocorticoid and antiprogestogenic activities, making it useful in various medical applications.

**Why the Correct Answer is Right**
Mifepristone is primarily an antiprogestin, which means it blocks the action of progesterone. This action is mediated through its high affinity for the progesterone receptor, preventing the hormone from binding and exerting its effects on the endometrium and the myometrium. By inhibiting progesterone's action, mifepristone induces uterine contractions, leading to its use in medical abortion and management of certain types of cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** Mifepristone is not a selective estrogen receptor modulator (SERM), which is a class of medications that can either block or mimic the effects of estrogen in the body.

**Option B:** Mifepristone is not a gonadotropin-releasing hormone (GnRH) agonist, which are medications that stimulate the release of GnRH, leading to a decrease in sex hormone production.

**Option C:** Mifepristone is not a nonsteroidal anti-inflammatory drug (NSAID), which is a class of medications that inhibit inflammation and pain by blocking the production of prostaglandins.

**Clinical Pearl / High-Yield Fact**
Mifepristone's antiprogestogenic effects make it useful in the management of conditions such as Cushing's syndrome, where excessive levels of cortisol can be life-threatening. However, it is essential to note that mifepristone is not a substitute for glucocorticoid replacement therapy in patients with adrenal insufficiency.

**Correct Answer:** D. Antiprogestin