**Core Concept:** Primary amenorrhea is the absence of menarche by age 16 in a girl who is developmentally mature. It can be due to genetic, chromosomal, or anatomical abnormalities. The examination findings mentioned in the question are typical of a 46XX karyotype, suggesting a genetic or chromosomal cause.

**Why the Correct Answer is Right:** The correct answer, Turner syndrome, is a genetic disorder caused by partial or complete monosomy of chromosome X. In Turner syndrome, the affected individual has two X chromosomes (45XX), but the presence of a normal X chromosome is sufficient for the diagnosis. The clinical presentation includes primary amenorrhea, short stature, webbed neck, characteristic facies, cubitus valgus, and cardiovascular abnormalities. In this case, the absence of a vagina and an absent uterus on ultrasound are consistent with the diagnosis of Turner syndrome.

**Why Each Wrong Option is Incorrect:**

A. 46XY Klinefelter syndrome (47XXY) is caused by an extra X chromosome and presents with gynecomastia, infertility, and delayed puberty. The absence of vagina and an absent uterus are not seen in this condition.

B. 46XO karyotype (monosomy of chromosome X) is associated with Turner syndrome and presents with short stature, webbed neck, and other features mentioned above. The absence of vagina and an absent uterus are not seen in this condition.

C. 45XO karyotype (monosomy of chromosome X) is associated with Turner syndrome and presents with short stature, webbed neck, and other features mentioned above. The absence of vagina and an absent uterus are not seen in this condition.

D. 45,XXY karyotype (disomy of chromosome Y) is associated with Klinefelter syndrome and presents with gynecomastia, infertility, and delayed puberty. The absence of vagina and an absent uterus are not seen in this condition.

**Clinical Pearl:** Turner syndrome is a common cause of primary amenorrhea in females, and early diagnosis is crucial for appropriate management and counselling of the patient and her family.