## **Core Concept**
The process of breast milk ejection, also known as the letdown reflex, is crucial for breastfeeding. It involves the contraction of myoepithelial cells surrounding the alveoli of the mammary glands, which helps in the ejection of milk. This reflex is primarily mediated by a specific hormone.

## **Why the Correct Answer is Right**
The hormone essential for breast milk ejection is **oxytocin**. When a baby suckles, it stimulates the nerve endings in the areola, which sends a signal to the hypothalamus. This signal leads to the release of oxytocin from the posterior pituitary gland. Oxytocin then acts on the myoepithelial cells of the mammary gland, causing them to contract and eject milk into the ducts, making it available for the baby to feed.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While prolactin is crucial for milk production (lactogenesis), it is not directly involved in the ejection of milk. Prolactin levels increase during pregnancy and remain high during lactation to support ongoing milk production.
- **Option B:** Growth hormone has various effects on growth and metabolism but is not directly involved in the milk ejection reflex.
- **Option C:** This option is not provided, but based on the context, we can assume it's not relevant to the milk ejection reflex.

## **Clinical Pearl / High-Yield Fact**
A memorable point for exams is that the milk ejection reflex can be inhibited by stress or pain, which can lead to difficulties with breastfeeding. This is because stress can inhibit the release of oxytocin, highlighting the importance of a calm and comfortable environment for effective breastfeeding.

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