## **Core Concept**
Inhibin is a protein complex that plays a crucial role in the regulation of the reproductive cycle by inhibiting the production of follicle-stimulating hormone (FSH) from the anterior pituitary gland. It is a key component in the feedback mechanism of the hypothalamic-pituitary-gonadal axis. Inhibin is produced by the granulosa cells in the ovaries of females and by the Sertoli cells in the testes of males.

## **Why the Correct Answer is Right**
The correct answer, **Graafian follicle (or mature follicle)**, is right because inhibin is primarily secreted by the granulosa cells of the Graafian follicle (or mature follicle) in the ovaries. This secretion is crucial for regulating FSH levels, which in turn affects the growth and maturation of follicles. The production of inhibin by the granulosa cells provides a negative feedback signal to the anterior pituitary gland to decrease FSH production.

## **Why Each Wrong Option is Incorrect**
- **Option A: Corpus luteum** - While the corpus luteum is involved in the production of various hormones like progesterone and estrogen after ovulation, it is not the primary source of inhibin.
- **Option B: Primary follicle** - The primary follicle does contain granulosa cells, but it is not the primary source of inhibin; rather, it is the more mature Graafian follicle.
- **Option D: Theca interna** - Theca interna cells produce androgens which are then converted to estrogens by the granulosa cells, but they are not primarily responsible for inhibin secretion.

## **Clinical Pearl / High-Yield Fact**
A key clinical correlation is that inhibin levels can be used as a marker in certain clinical conditions. For instance, inhibin B levels are often used as a marker of ovarian reserve. Low levels of inhibin B have been associated with decreased ovarian reserve and poor response to ovarian stimulation.

## **Correct Answer: C. Graafian follicle**