## **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 controlling FSH secretion. Inhibin is produced by the gonads and has two main types: inhibin A and inhibin B.

## **Why the Correct Answer is Right**
The correct answer, **Graafian follicle**, is right because inhibin is primarily secreted by the granulosa cells of the Graafian (mature) follicles in the ovaries. In males, it is produced by the Sertoli cells of the testes. Inhibin A is predominantly produced by the dominant follicle in the ovary and by the Sertoli cells in the testis, while inhibin B is produced by the granulosa cells of the antral follicles. Both types of inhibin feedback to the anterior pituitary gland to decrease FSH production.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, although the corpus luteum is involved in the menstrual cycle and produces hormones like progesterone, it is not the primary source of inhibin.
- **Option B:** This option might seem plausible due to the role of the Sertoli cells in producing inhibin in males, but the question seems to focus on female reproductive structures given the mention of "Graafian follicle."
- **Option C:** This option is incorrect because the theca interna, while involved in follicular development and producing androgens that are converted to estrogens by the granulosa cells, is not the primary source of inhibin.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that inhibin B levels are often used as a marker of ovarian reserve. Low levels of inhibin B are associated with decreased ovarian reserve and can be an indicator of diminished fertility. This is particularly relevant in the context of assisted reproductive technologies.

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