**Question:** All of the following agents act through nuclear receptors EXCEPT:

A. Progesterone
B. Estrogen
C. Glucocorticoids
D. Gonadotropin-releasing hormone (GnRH)

**Core Concept:** Nuclear receptors are a class of proteins that function as transcription factors within the cell nucleus, binding to specific ligands (hormones, steroids, or vitamins) and regulating gene expression, ultimately leading to physiological responses.

**Why the Correct Answer is Right:** Gonadotropin-releasing hormone (GnRH) is a precursor to the gonadotropins (luteinizing hormone and follicle-stimulating hormone) that act on the pituitary gland to regulate gonadal function. It does not act through nuclear receptors, making D the correct answer.

**Why Each Wrong Option is Incorrect:**

A. Progesterone: This steroid hormone acts through the progesterone receptor and modulates various physiological processes, including endometrial development, placental function, and immune system modulation.

B. Estrogen: Estrogen is another steroid hormone that acts through the estrogen receptor to regulate various cellular processes, including cell proliferation, differentiation, and gene expression.

C. Glucocorticoids: Glucocorticoids, such as cortisol, act through glucocorticoid receptors to regulate immune response, inflammation, and energy homeostasis. They also play a crucial role in the pathogenesis of various diseases like Cushing's disease and Addison's disease.

**Clinical Pearl:** Nuclear receptors are essential components of hormone action. Understanding their role in various physiological processes aids in interpreting clinical scenarios involving hormone imbalances or receptor disorders like estrogen receptor alpha (ERα) or beta (ERβ) mutations in breast or endometrial cancers.

**Correct Answer:** D (Gonadotropin-releasing hormone)

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**Core Concept:** Nuclear receptors are a class of proteins that function as transcription factors within the cell nucleus, binding to specific ligands (hormones, steroids, or vitamins) and regulating gene expression, ultimately leading to physiological responses.

**Why the Correct Answer is Right:** Gonadotropin-releasing hormone (GnRH) is a neuropeptide that triggers the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the pituitary gland to regulate gonadal function. GnRH does not act through nuclear receptors, making D the correct answer.

**Why Each Wrong Option is Incorrect:**

A. Progesterone: Progesterone, like estrogen, is a steroid hormone that acts through the progesterone receptor (PR) to regulate uterine smooth muscle contraction and endometrial growth.

B. Estrogen: Estrogen is a steroid hormone that acts through the estrogen receptor (ER) to modulate cellular processes like cell proliferation, differentiation, and gene expression in various tissues, particularly in breast and endometrial cancers.

C.