**Core Concept**
Ulipristal acetate is a selective progesterone receptor modulator (SPRM) used in the management of emergency contraception and the treatment of uterine fibroids. It works by binding to the progesterone receptor, thereby inhibiting progesterone's action on the endometrium.

**Why the Correct Answer is Right**
Ulipristal acetate's mechanism of action involves competitive binding to the progesterone receptor, which is a key regulator of the menstrual cycle. By inhibiting progesterone's action, ulipristal acetate prevents ovulation and thickening of the endometrium, thereby preventing implantation of a fertilized egg. This makes it an effective emergency contraceptive.

**Why Each Wrong Option is Incorrect**
**Option A:** Ulipristal acetate is not an estrogen receptor modulator, as it does not affect the estrogen receptor pathway.
**Option B:** While ulipristal acetate does affect the reproductive system, it is not a gonadotropin-releasing hormone (GnRH) agonist, which would suppress gonadotropin release.
**Option C:** Ulipristal acetate is not a selective estrogen receptor modulator (SERM), which would act on the estrogen receptor.

**Clinical Pearl / High-Yield Fact**
Ulipristal acetate is a useful alternative to traditional emergency contraceptives like levonorgestrel, as it is effective for up to 5 days after unprotected sex and has a lower risk of side effects.

**Correct Answer: C. Selective progesterone receptor modulator (SPRM)**