**Core Concept**
Implanon is a type of long-acting reversible contraceptive (LARC) that uses a unique mechanism to prevent pregnancy. It is a subdermal implant that releases a synthetic progestin hormone, which affects the reproductive system to prevent ovulation.

**Why the Correct Answer is Right**
The synthetic progestin hormone in Implanon is etonogestrel, a third-generation progestin with a high molecular weight. This allows it to be slowly released over several years, providing effective contraception for up to 3 years. Etonogestrel works by inhibiting the luteinizing hormone (LH) surge, which is necessary for ovulation. By preventing the LH surge, etonogestrel effectively suppresses ovulation, preventing the release of an egg from the ovary.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because etonogestrel is a synthetic progestin, not a synthetic estrogen.
* **Option B:** This option is incorrect because etonogestrel is not a type of estrogen, and it does not work by stimulating the growth of the endometrium.
* **Option C:** This option is incorrect because etonogestrel is a progestin, not a GnRH agonist, and it does not work by suppressing the pituitary gland.

**Clinical Pearl / High-Yield Fact**
One of the key benefits of Implanon is its long duration of action, which makes it a convenient option for women who want a reliable and hassle-free form of contraception. Implanon is also a good option for women who have difficulty remembering to take daily birth control pills.

**Correct Answer: C. Etonogestrel**