**Core Concept:** Estrogen is a hormone that plays a crucial role in the development and maintenance of the female reproductive system and secondary sexual characteristics. In the mammary gland, estrogen stimulates growth, differentiation, and secretory function.

**Why the Correct Answer is Right:** Estrogen exerts its effects through binding to estrogen receptors (ERs), which are proteins that act as transcription factors within the nucleus of target cells. The three main types of estrogen receptors are ERα, ERβ, and GPR30. In the context of the mammary gland, estrogen mainly binds to ERα, which then activates transcription factors and leads to changes in gene expression. This results in the stimulation of cellular proliferation, differentiation, and milk production.

**Why Each Wrong Option is Incorrect:**

A. This statement is partially true. Estrogen does play a role in the development and maintenance of the mammary gland, but the other options are also relevant.

B. While estrogen is involved in mammary gland development, this statement is incorrect as the mammary gland is not specifically a "sexual organ."

C. While estrogen does stimulate the mammary gland, this statement is incorrect because the hormone progesterone also plays a significant role in the gland's development and function.

D. This statement is incorrect as estrogen does stimulate the mammary gland, though it is not the only hormone responsible for its function.

**Clinical Pearl:** Understanding the complex hormonal milieu that influences the development and function of the mammary gland is essential for understanding various clinical aspects, including breast cancer risk, pregnancy-related changes in the breast, and lactation.

**Correct Answer:** D. Estrogen is NOT the only hormone responsible for the function of the mammary gland. Progesterone also plays a significant role, acting synergistically with estrogen to promote mammary gland development and function. Estrogen and progesterone are part of the hormonal cascade that leads to lactation during pregnancy.