**Core Concept**
The underlying principle being tested is the evaluation of primary amenorrhea, specifically the assessment of the hypothalamic-pituitary-ovarian axis and the responsiveness of the uterus to hormonal stimulation. This involves understanding the role of **estrogen** and **progesterone** in the menstrual cycle.

**Why the Correct Answer is Right**
The patient's normal secondary sex characteristics suggest adequate **estrogen** levels, indicating functioning ovaries. The withdrawal bleeding after progesterone administration confirms that the uterus is responsive to hormonal stimulation, suggesting an intact **hypothalamic-pituitary-ovarian axis**. This points towards a problem with the outflow tract rather than a hormonal imbalance.

**Why Each Wrong Option is Incorrect**
**Option A:** This would imply a lack of **estrogen**, contradicting the presence of normal secondary sex characteristics.
**Option B:** This suggests a problem with the **hypothalamic-pituitary** axis, which is not supported by the patient's response to progesterone.
**Option C:** This implies an issue with **ovarian** function, which is not consistent with the normal development of secondary sex characteristics.

**Clinical Pearl / High-Yield Fact**
A key point in evaluating primary amenorrhea is the progesterone withdrawal test, which helps differentiate between issues of hormonal imbalance and anatomical obstruction.

**Correct Answer:** D. Anatomical obstruction.