**Core Concept**
Estradiol is a major estrogen sex hormone, essential for maintaining female reproductive and sexual health. Its reduction can lead to various physiological changes, primarily affecting the female reproductive system.

**Why the Correct Answer is Right**
A decrease in estradiol levels can lead to a decrease in the growth of the endometrium, resulting in atrophic vaginitis. This is due to the reduced proliferative effect of estradiol on the endometrial cells. Additionally, reduced estradiol levels can lead to a decrease in the synthesis of progesterone receptors, making the endometrium less responsive to progesterone. This can disrupt the normal menstrual cycle and lead to irregular menstrual bleeding.

**Why Each Wrong Option is Incorrect**
**Option A:** Decreased bone density - Incorrect because estradiol plays a crucial role in maintaining bone density in women, and its reduction can lead to an increase in bone resorption and decreased bone formation, resulting in osteoporosis.
**Option B:** Increased risk of cardiovascular disease - Incorrect because estradiol has a protective effect on the cardiovascular system, and its reduction can lead to an increased risk of cardiovascular disease.
**Option C:** Reduced libido - Incorrect because estradiol has a role in regulating libido, and its reduction can lead to decreased libido in women.

**Clinical Pearl / High-Yield Fact**
Reduced estradiol levels can lead to a range of symptoms, including hot flashes, mood changes, and vaginal dryness. These symptoms can be managed with hormone replacement therapy (HRT) or other treatments.

**Correct Answer: C. Reduced libido.**