The commonest cause of primary Amenorrhoea is
**Core Concept**
Primary amenorrhea refers to the absence of menarche by the age of 16 in individuals with normal growth and secondary sexual characteristics, or the absence of menstruation by the age of 14 in those with delayed growth and development. This condition is often associated with anatomical, hormonal, or genetic abnormalities.
**Why the Correct Answer is Right**
The commonest cause of primary amenorrhea is Turner syndrome, which accounts for approximately 15-20% of cases. This condition is characterized by the absence of one X chromosome (45,X), leading to ovarian dysgenesis and subsequent estrogen deficiency. The lack of estrogen production prevents the development of secondary sexual characteristics and menstruation. The absence of the ovaries also disrupts the hypothalamic-pituitary-gonadal axis, further contributing to the amenorrheic state.
**Why Each Wrong Option is Incorrect**
**Option A:** Congenital adrenal hyperplasia is a rare cause of primary amenorrhea, often presenting with virilization and hormonal imbalances.
**Option B:** Kallmann syndrome is a genetic disorder that affects the development of the hypothalamus, leading to delayed or absent puberty, but it is not the most common cause of primary amenorrhea.
**Option C:** Rokitansky syndrome (Mayer-Rokitansky-Küster-Hauser syndrome) is a congenital disorder characterized by the absence or underdevelopment of the vagina and uterus, but it is less common than Turner syndrome.
**Clinical Pearl / High-Yield Fact**
Turner syndrome is often associated with short stature, heart defects, and infertility, and can be diagnosed through karyotyping or molecular genetic testing.
**Correct Answer: A. Turner syndrome.**