**Core Concept**
Combined oral contraceptive pills (COCs) are a type of hormonal contraception that contain estrogen and progesterone. They work by inhibiting ovulation, thickening cervical mucus to prevent sperm penetration, and altering the endometrium to prevent implantation.

**Why the Correct Answer is Right**
Combined oral pills do offer some benefits, including reducing the risk of ovarian and endometrial cancer, regulating menstrual cycles, and alleviating symptoms of premenstrual syndrome. However, they can also increase the risk of venous thromboembolism (VTE), which includes deep vein thrombosis (DVT) and pulmonary embolism (PE). This increased risk is due to the estrogen component, which can lead to blood clot formation. The incidence of VTE is higher in women taking COCs, especially in those with a history of clotting disorders or other risk factors.

**Why Each Wrong Option is Incorrect**
**Option A:** Menorrhagia is actually a potential side effect of combined oral pills, as they can alter menstrual flow and lead to irregular or heavy bleeding in some women.
**Option B:** Combined oral pills have been shown to reduce the risk of benign breast disease, including fibroadenoma and fibrocystic changes.
**Option C:** Pelvic inflammatory disease (PID) is not directly prevented by combined oral pills. In fact, the risk of PID may be increased in women using COCs, especially if they have a history of sexually transmitted infections.

**Clinical Pearl / High-Yield Fact**
Remember that combined oral pills are not a cure-all for menstrual disorders. While they can alleviate some symptoms, they may also introduce new risks and side effects, such as VTE.

**✓ Correct Answer: D. Venous thromboembolism**