**Core Concept**
Mirena is a type of intrauterine device (IUD) that releases a small amount of levonorgestrel (LNG), a synthetic progestin, to prevent pregnancy. The release of LNG from Mirena helps to create an environment that is unfavorable for implantation and fertilization.

**Why the Correct Answer is Right**
Mirena releases 20 micrograms of LNG per day, which is a very small amount compared to oral contraceptive pills. This slow release of LNG helps to maintain a stable concentration of the hormone in the uterus, thereby providing effective contraception. The LNG released from Mirena also causes the cervical mucus to thicken, making it difficult for sperm to reach the egg.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because Mirena does not release 10 micrograms of LNG per day. This is a lower dose than what is actually released.
**Option B:** This is incorrect because Mirena does not release 30 micrograms of LNG per day. This is a higher dose than what is actually released.
**Option C:** This is incorrect because Mirena does not release 40 micrograms of LNG per day. This is a much higher dose than what is actually released.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Mirena is a long-acting reversible contraceptive (LARC) that can provide effective contraception for up to 5 years. This makes it a great option for women who want a convenient and reliable form of birth control.

**Correct Answer: D. 20 micrograms**