**Core Concept**
Oral contraceptives (OCs) are a combination of estrogen and progestin that work by inhibiting the release of gonadotropin-releasing hormone (GnRH), leading to decreased production of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). This results in a decrease in estrogen production, thereby preventing ovulation.

**Why the Correct Answer is Right**
The correct answer is related to the side effects of OCs, which can be divided into two categories: systemic and localized. Systemic side effects include nausea, vomiting, breast tenderness, and mood changes. Localized side effects include breakthrough bleeding, amenorrhea, and changes in cervical mucus. The correct answer is a side effect that is not commonly associated with OCs.

**Why Each Wrong Option is Incorrect**
**Option A:** Weight gain is a well-documented side effect of OCs, particularly those containing estrogen. The increased estrogen levels can lead to water retention and fat redistribution, resulting in weight gain.

**Option B:** Mood changes, including depression, anxiety, and irritability, are common side effects of OCs. The exact mechanism is not fully understood, but it is thought to be related to the effects of estrogen on neurotransmitters such as serotonin.

**Option C:** Breakthrough bleeding is a common side effect of OCs, particularly during the initial months of use. It is usually due to the body adjusting to the new hormonal regimen.

**Clinical Pearl / High-Yield Fact**
It's essential to note that OCs can also have a protective effect against certain conditions, such as ovarian and endometrial cancer, and can help regulate menstrual cycles in women with polycystic ovary syndrome (PCOS).

**Correct Answer: D.**