**Core Concept**
Oral contraceptive (OC) pills are a type of hormonal medication that regulate ovulation and prevent pregnancy. They contain a combination of estrogen and progesterone, which can cause various side effects due to their hormonal activity.

**Why the Correct Answer is Right**
The side effects of OC pills are primarily due to the hormonal changes they induce in the body. Estrogen can cause water retention, leading to weight gain and bloating, while progesterone can cause breast tenderness and mood changes. Additionally, OC pills can affect liver function, leading to changes in liver enzymes, and can also increase the risk of blood clots due to their effect on blood coagulation factors.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a known side effect of OC pills. In fact, OC pills can actually help regulate menstrual cycles and reduce the risk of certain pelvic disorders.
**Option B:** This option is incorrect because while OC pills can cause nausea and vomiting, they are not a common side effect and are more often associated with the initial use of the medication.
**Option C:** This option is incorrect because OC pills can actually help reduce the risk of endometrial cancer, rather than cause it.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that OC pills can affect liver function and increase the risk of blood clots, particularly in women who smoke or have a family history of clotting disorders.

**Correct Answer:** D.