**Core Concept**
Primary amenorrhea refers to the absence of menstruation in a woman by the age of 16 years, in the presence of normal secondary sexual characteristics. It is often due to underlying anatomical, hormonal, or genetic abnormalities that prevent normal menstrual cycling.

**Why the Correct Answer is Right**
Sheehan's syndrome is a condition characterized by postpartum necrosis of the pituitary gland, leading to hypopituitarism. This results in decreased production of gonadotropins, specifically follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which are essential for ovarian function and menstruation. Although Sheehan's syndrome can cause amenorrhea, it is typically secondary amenorrhea, occurring in a woman who was previously menstruating.

**Why Each Wrong Option is Incorrect**
**Option B:** Kallmann's syndrome is a genetic disorder that affects the hypothalamus, leading to delayed or absent puberty, including primary amenorrhea. It is characterized by anosmia (loss of smell) and hypogonadotropic hypogonadism, resulting from a deficiency of gonadotropin-releasing hormone (GnRH).

**Option C:** Mayer Rokitansky Koster Hauser (MRKH) syndrome is a congenital disorder that affects the development of the Müllerian ducts, resulting in the absence or underdevelopment of the uterus and upper vagina. This condition is a classic example of primary amenorrhea.

**Option D:** Turner syndrome is a genetic disorder that affects females, characterized by the presence of only one X chromosome (45,X). It often results in short stature, ovarian dysgenesis, and primary amenorrhea due to the lack of ovarian function.

**Clinical Pearl / High-Yield Fact**
Remember that primary amenorrhea is a critical diagnostic clue for underlying anatomical, hormonal, or genetic abnormalities. A thorough evaluation, including pelvic examination, imaging studies, and hormonal assays, is essential to identify the underlying cause.

**✓ Correct Answer: A. Sheehan's syndrome**