## **Core Concept**
The regulation of follicle-stimulating hormone (FSH) is crucial in the female reproductive system, impacting ovarian follicle development and estrogen production. FSH secretion is primarily modulated by feedback mechanisms involving sex hormones and inhibins. **Estradiol** and **inhibin** are key regulators that provide negative feedback to control FSH levels.

## **Why the Correct Answer is Right**
Inhibin is a protein complex produced by the granulosa cells in the ovaries of females and by the Sertoli cells in the testes of males. It plays a critical role in the regulation of FSH secretion. **Inhibin selectively inhibits the production and secretion of FSH from the anterior pituitary gland**, without affecting the secretion of luteinizing hormone (LH) or other hormones. This makes inhibin a powerful and specific inhibitor of FSH.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While estradiol does provide negative feedback on FSH secretion, the question asks for another factor. Estradiol's role is well established, but the question seeks an additional regulator.
- **Option B:** This option is not specified, but based on the context, it seems irrelevant or not applicable as an inhibitor of FSH.
- **Option D:** Similarly, this option is unspecified and does not directly relate to known potent inhibitors of FSH.

## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is the role of **inhibin** and **estradiol** in regulating FSH levels. In conditions like premature ovarian failure or menopause, the decrease in **inhibin** and **estradiol** levels leads to an increase in FSH secretion. This feedback mechanism is critical in the diagnosis and management of various reproductive disorders.

## **Correct Answer:** C. Inhibin.