## **Core Concept**
The ejection of milk, also known as the letdown reflex, is a crucial process in lactation. It involves the contraction of myoepithelial cells surrounding the alveoli of the mammary gland, which leads to the ejection of milk. This process is primarily mediated by a neuroendocrine reflex.

## **Why the Correct Answer is Right**
The correct answer, **Oxytocin**, is right because oxytocin is the hormone responsible for stimulating the contraction of myoepithelial cells around the mammary gland alveoli. This contraction results in the ejection of milk from the alveoli into the ducts, making it available for the infant. Oxytocin is released from the posterior pituitary gland in response to suckling, which is detected by sensory receptors in the nipple.

## **Why Each Wrong Option is Incorrect**
- **Option A: Prolactin** - Prolactin is involved in the initiation of milk production (lactogenesis) but not directly in the ejection of milk. It stimulates the growth of the mammary glands and the production of milk.
- **Option B: Luteinizing hormone (LH)** - LH plays a critical role in ovulation and the maintenance of the corpus luteum but is not directly involved in milk ejection.
- **Option C: Progesterone** - Progesterone is essential for the preparation of the breast for milk production during pregnancy but does not play a direct role in the ejection of milk.

## **Clinical Pearl / High-Yield Fact**
A memorable point for exams is that the letdown reflex, or milk ejection reflex, can be conditioned to auditory or visual cues, which is why some mothers may experience milk letdown when hearing a baby cry or seeing their baby. This reflex is crucial for successful breastfeeding.

## **Correct Answer:** D. Oxytocin