**Core Concept**
Gonadotropin-releasing hormone (GnRH) is a decapeptide hormone secreted by the hypothalamus, which plays a crucial role in regulating the reproductive axis by controlling the secretion of gonadotropins, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) from the anterior pituitary gland.

**Why the Correct Answer is Right**
GnRH acts on the anterior pituitary gland by binding to GnRH receptors, which triggers a series of intracellular signaling pathways that ultimately lead to the release of LH and FSH. The release of LH and FSH stimulates the gonads to produce sex hormones, such as testosterone in males and estrogen in females. GnRH also has a negative feedback effect on its own secretion, which is mediated by the sex hormones.

**Why Each Wrong Option is Incorrect**
**Option A:** GnRH is not a peptide hormone; it is a decapeptide hormone.
**Option B:** GnRH does not primarily regulate the secretion of aldosterone; it regulates the secretion of gonadotropins.
**Option C:** GnRH does not act on the adrenal cortex; it acts on the anterior pituitary gland.

**Clinical Pearl / High-Yield Fact**
GnRH is used clinically in the form of GnRH agonists and antagonists, which are used to treat conditions such as endometriosis, uterine fibroids, and prostate cancer.

**Correct Answer: D. Decapeptide hormone.**