**Core Concept**
Mirena, also known as the Levonorgestrel-Releasing Intrauterine Contraceptive Device (LNG IUCD), is a local progestin-releasing contraceptive system used to prevent pregnancy. It works by releasing a steady dose of levonorgestrel, a synthetic progestin, which creates a local hormonal environment that inhibits ovulation and fertilization.

**Why the Correct Answer is Right**
The correct answer is B. 5 years because Mirena's efficacy in preventing pregnancy is well-documented over a period of 5 years. The sustained release of levonorgestrel from the device leads to a steady decrease in menstrual bleeding, making it an effective treatment for heavy menstrual bleeding. Studies have shown that Mirena remains effective for up to 5 years, after which time its efficacy may decrease.

**Why Each Wrong Option is Incorrect**
**Option A:** 1 year is too short a duration for Mirena's replacement, as its efficacy is well-established beyond this timeframe.
**Option C:** 3 years is also not the correct answer, as Mirena's manufacturer recommends replacement every 5 years for optimal efficacy.
**Option D:** 6 months is far too short a duration for Mirena's replacement, as its local hormonal effects take time to develop and maintain contraceptive efficacy.

**Clinical Pearl / High-Yield Fact**
Mirena is an effective treatment for heavy menstrual bleeding (HMB), with up to 90% of users experiencing a significant reduction in menstrual bleeding within the first year of use. This makes it an excellent option for patients with HMB.

✓ Correct Answer: B. 5 years