## **Core Concept**
The question tests knowledge of pituitary hormones and their classification, specifically focusing on the identification of an opioid peptide. Pituitary hormones are produced by the pituitary gland and regulate various bodily functions. Opioid peptides are endogenous peptides that have opioid-like effects.

## **Why the Correct Answer is Right**
The correct answer, **β-endorphin**, is indeed an opioid peptide produced by the pituitary gland. β-endorphins are synthesized from the pro-opiomelanocortin (POMC) precursor and play a significant role in pain modulation and stress response. They act on opioid receptors in the brain to produce analgesia and feelings of well-being.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **ACTH (Adrenocorticotropic hormone)** is produced by the pituitary gland and is also derived from the POMC precursor like β-endorphin, it does not have opioid peptide activity. ACTH primarily stimulates the secretion of cortisol from the adrenal cortex.
- **Option B:** **TSH (Thyroid-stimulating hormone)** regulates the endocrine function of the thyroid gland but is not an opioid peptide. It does not have structural or functional similarities to opioid peptides.
- **Option D:** **FSH (Follicle-stimulating hormone)** is crucial for the growth and maturation of ovarian follicles in females and spermatogenesis in males. Like TSH, it is not an opioid peptide.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that β-endorphins are not only involved in pain modulation but also in stress response and are released in response to stress. Their opioid activity can lead to euphoria and are sometimes referred to as "natural painkillers."

## **Correct Answer:** . β-endorphin.