**Core Concept**
Ullipristal acetate is a selective progesterone receptor modulator (SPRM) used in emergency contraception and the treatment of uterine fibroids. It works by binding to the progesterone receptor, which influences the growth and maintenance of the endometrium.

**Why the Correct Answer is Right**
Ullipristal acetate binds to the progesterone receptor, preventing the endometrial transformation necessary for implantation of a fertilized egg, thereby acting as an emergency contraceptive. It also has antiproliferative effects on the endometrium, making it useful in the treatment of uterine fibroids. This mechanism is distinct from other forms of emergency contraception, such as the combined oral contraceptive pill or levonorgestrel, which primarily work by inhibiting or delaying ovulation.

**Why Each Wrong Option is Incorrect**
**Option A:** Ullipristal acetate is not an estrogen receptor modulator, as it primarily targets the progesterone receptor.
**Option B:** Ullipristal acetate is not a gonadotropin-releasing hormone (GnRH) agonist, as it does not primarily work through the hypothalamic-pituitary-gonadal axis.
**Option C:** Ullipristal acetate is not a selective estrogen receptor modulator (SERM), as it is specifically designed to target the progesterone receptor.

**Clinical Pearl / High-Yield Fact**
Ullipristal acetate can be used up to 120 hours (5 days) after unprotected sex, making it a more flexible emergency contraceptive option compared to levonorgestrel, which is most effective when used within 72 hours.

**Correct Answer:** C. Uterine fibroid treatment