**Core Concept**
Mirena is a type of levonorgestrel-releasing intrauterine system (LNG-IUS) used for contraception and management of heavy menstrual bleeding. It works by releasing a steady dose of the progestin levonorgestrel, which has a local effect on the endometrium.

**Why the Correct Answer is Right**
The mechanism of action of Mirena involves the release of levonorgestrel, which binds to progesterone receptors in the endometrium. This binding causes thinning of the endometrium, leading to reduced menstrual bleeding. Additionally, the progestin effect on the endometrium makes it less receptive to implantation of a fertilized egg, providing contraceptive protection.

**Why Each Wrong Option is Incorrect**
**Option A:** Mirena releases estrogen, which is incorrect as it is a progestin-only system.
**Option B:** Mirena is used primarily for emergency contraception, which is incorrect as it is used for long-term contraception and management of heavy menstrual bleeding.
**Option C:** Mirena is a type of estrogen-releasing IUD, which is incorrect as it is a progestin-only system.

**Clinical Pearl / High-Yield Fact**
Mirena is a good option for women with heavy menstrual bleeding due to its local effect on the endometrium, reducing menstrual blood loss by up to 90% in some cases. However, it is essential to rule out underlying bleeding disorders or other causes of heavy bleeding before prescribing Mirena.

**Correct Answer: B. Mirena is used primarily for long-term contraception and management of heavy menstrual bleeding.**