## **Core Concept**
Implanon is a type of contraceptive implant that releases a progestin hormone to prevent pregnancy. It is a single rod implant that provides contraception for up to three years. The primary mechanism of action is through the release of a steady dose of progestin.

## **Why the Correct Answer is Right**
The correct answer, **Etonogestrel**, is a type of progestin used in Implanon. Etonogestrel is a third-generation progestin that works by inhibiting ovulation, thickening cervical mucus to prevent sperm penetration, and altering the endometrium to prevent implantation. It is released from the Implanon rod at a steady rate over three years.

## **Why Each Wrong Option is Incorrect**
* **Option A: Estradiol** - This is incorrect because Implanon is a progestin-only contraceptive. Estradiol is a form of estrogen, which is not the primary hormone released by Implanon.
* **Option B: Levonorgestrel** - This is incorrect because while Levonorgestrel is a type of progestin used in some contraceptives (like the Mirena IUD and certain oral contraceptives), it is not the progestin used in Implanon.
* **Option C: Norethindrone** - This is incorrect because Norethindrone is a first-generation progestin used in some older contraceptives and is not associated with Implanon.
* **Option D: Medroxyprogesterone** - This is incorrect because Medroxyprogesterone is a different type of progestin used in depot injections (like Depo-Provera) and is not used in Implanon.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that Implanon is effective for up to three years and its removal is recommended after this period for continued contraceptive efficacy. Additionally, Implanon's insertion and removal require specific training, making it a procedure often performed in clinical settings.

## **Correct Answer: E. Etonogestrel**