**Core Concept**
Milk ejection, also known as the letdown reflex, is a physiological process that enables the release of milk from the mammary glands during lactation. This process is regulated by the neuroendocrine system, involving the hypothalamus, pituitary gland, and mammary glands.

**Why the Correct Answer is Right**
Oxytocin is a hormone released by the posterior pituitary gland, which plays a crucial role in milk ejection. When a suckling infant stimulates the nipple, the afferent nerve fibers transmit signals to the hypothalamus, which then releases oxytocin into the bloodstream. Oxytocin binds to oxytocin receptors in the myoepithelial cells surrounding the alveoli of the mammary glands, causing contraction and milk ejection. This process is essential for the successful feeding of infants during lactation.

**Why Each Wrong Option is Incorrect**
**Option B:** Growth hormone, released by the anterior pituitary gland, stimulates cell growth and reproduction but does not play a direct role in milk ejection.
**Option C:** Follicle-stimulating hormone (FSH), also released by the anterior pituitary gland, regulates follicular growth in the ovaries and is not involved in milk ejection.
**Option D:** Luteinizing hormone (LH), released by the anterior pituitary gland, controls ovulation and is not directly involved in milk ejection.

**Clinical Pearl / High-Yield Fact**
Oxytocin has a critical role not only in milk ejection but also in uterine contractions during childbirth, making it a vital hormone for both lactation and parturition.

**✓ Correct Answer: A. Oxytocin**