**Core Concept**
Nexplanon, a progestin-only subdermal implant, is a highly effective contraceptive method. It works by releasing a steady amount of etonogestrel, a potent progestin, which affects the menstrual cycle and ovulation.

**Why the Correct Answer is Right**
Nexplanon primarily functions by suppressing ovulation, thereby preventing fertilization. It achieves this by releasing etonogestrel, which acts on the hypothalamus to decrease gonadotropin-releasing hormone (GnRH) secretion, ultimately leading to reduced luteinizing hormone (LH) and follicle-stimulating hormone (FSH) production. This results in ovulation suppression, making it an effective contraceptive method. Additionally, Nexplanon induces endometrial atrophy, making it difficult for a fertilized egg to implant. It also increases cervical mucus viscosity, thereby creating a physical barrier that prevents sperm from reaching the egg.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as Nexplanon primarily suppresses ovulation, but it's not the only mechanism of action. However, this is not the correct answer as it is still true about Nexplanon.
**Option B:** Incorrect, as Nexplanon does induce endometrial atrophy, but this is not the reason why it's the correct answer.
**Option C:** Incorrect, as Nexplanon is actually a progestin-only subdermal implant, not an estrogen subdermal implant.
**Option D:** Incorrect, as Nexplanon does increase cervical mucus viscosity, but this is not the reason why it's the correct answer.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Nexplanon is a progestin-only contraceptive, which means it does not contain estrogen, making it an ideal option for women with a history of estrogen-related side effects or contraindications.

**Correct Answer:**
✓ Correct Answer: C. Estrogen subdermal implant.