**Core Concept:** In females, estrogen plays a crucial role in initiating and regulating various pubertal events. Estrogen is produced by the developing ovaries and stimulates the growth and development of secondary sexual characteristics, such as breast development, menstruation, and increased body fat.

**Why the Correct Answer is Right:** The correct answer, **D**, refers to the onset of menstruation (menarche) as not being estrogen-dependent. Menarche occurs due to a combination of hormonal factors, including estrogen and follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary gland. Although estrogen contributes to the process, menarche is primarily regulated by the hypothalamic-pituitary-ovarian (HPO) axis and the surge of FSH and LH, which trigger follicle development and ovulation.

**Why Each Wrong Option is Incorrect:**

A. **Option A (Breast Development):** Estrogen is the primary hormone responsible for breast development during puberty. It stimulates the growth and development of breast tissue, including the enlargement of the areola and the development of the nipple and mammary glands.

B. **Option B (Increased Body Fat):** Estrogen is involved in the regulation of body fat distribution during puberty. It promotes the accumulation of body fat in the gluteal-femoral region, while inhibiting fat deposition in the face and upper body.

C. **Option C (Menstruation):** As mentioned earlier, menstruation is primarily regulated by the surge of FSH and LH, released from the hypothalamus and pituitary gland respectively. Although estrogen plays a role in the regulation of the HPO axis, it is not solely responsible for the onset of menstruation.

**Clinical Pearl:** Understanding the interplay between different hormones and their roles in puberty can help medical students and practitioners interpret clinical scenarios involving secondary sexual characteristics and menstrual irregularities. A thorough understanding of the HPO axis and the actions of FSH, LH, and estrogen is essential in diagnosing and treating conditions like delayed puberty, premature menopause, or other endocrine disorders affecting the onset of puberty or its progression.