**Core Concept**
Primary amenorrhea refers to the condition where a woman has not experienced her first menstrual period by the age of 16, despite normal secondary sexual characteristics. Eugonadism indicates that the ovaries are functioning normally, producing both estrogen and progesterone.

**Why the Correct Answer is Right**
Primary amenorrhea with eugonadism is often caused by anatomical or structural abnormalities in the reproductive tract. These can include imperforate hymen, Müllerian agenesis, and vaginal atresia. In these conditions, the ovaries are functioning normally, but the absence of a normal reproductive tract prevents menstrual flow.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Turner syndrome is a chromosomal abnormality that affects ovarian function, leading to hormonal imbalances and secondary amenorrhea.
* **Option B:** This option is incorrect because androgen insensitivity syndrome is a condition where the body is unable to respond to androgens, leading to a lack of secondary sexual characteristics and primary amenorrhea due to hormonal imbalances.
* **Option C:** This option is incorrect because congenital adrenal hyperplasia can cause hormonal imbalances and secondary amenorrhea due to excessive androgen production.

**Clinical Pearl / High-Yield Fact**
It's essential to note that primary amenorrhea can be caused by a wide range of conditions, and a thorough physical examination and hormonal evaluation are necessary to determine the underlying cause.

**Correct Answer: A. Turner syndrome is a chromosomal abnormality that affects ovarian function, leading to hormonal imbalances and secondary amenorrhea.