**Core Concept**
The most common endocrine disorder in reproductive-aged women involves hormonal imbalances affecting menstrual cycles, fertility, and overall reproductive health. Polycystic ovary syndrome (PCOS) is a complex endocrine disorder characterized by ovulatory dysfunction, hyperandrogenism, and polycystic ovarian morphology.

**Why the Correct Answer is Right**
PCOS is a multifactorial disorder caused by insulin resistance, genetic predisposition, and environmental factors. Women with PCOS often experience irregular menstrual cycles, hirsutism, acne, and obesity. The pathophysiology involves increased androgen production by the ovarian stroma, leading to anovulation and ovulatory dysfunction. The insulin resistance component contributes to hyperinsulinemia, which further exacerbates androgen production.

**Why Each Wrong Option is Incorrect**
**Option A:** Hyperprolactinemia is a common endocrine disorder in reproductive-aged women, but it is not the most common. Hyperprolactinemia often presents with amenorrhea, galactorrhea, and infertility.
**Option B:** Congenital adrenal hyperplasia (CAH) is a rare genetic disorder affecting adrenal steroidogenesis, leading to virilization and hormonal imbalances. It is not the most common endocrine disorder in reproductive-aged women.
**Option C:** Thyroid disorders, such as hypothyroidism, can affect reproductive health but are not the most common endocrine disorder in this age group.

**Clinical Pearl / High-Yield Fact**
PCOS is often associated with insulin resistance, metabolic syndrome, and an increased risk of gestational diabetes and type 2 diabetes.

**Correct Answer:** C.