**Core Concept**
The girl's clinical presentation suggests Turner syndrome, a genetic disorder characterized by partial or complete absence of one X chromosome. This condition is associated with various physical and developmental abnormalities, including short stature, neck webbing, and gonadal dysgenesis leading to sexual infantilism.

**Why the Correct Answer is Right**
Turner syndrome is caused by the loss or alteration of one X chromosome, resulting in 45,X instead of the typical 46,XX or 46,XY karyotype. The presence of coarctation of the aorta, a congenital heart defect, is also a common feature of Turner syndrome. The chromosomal analysis would likely demonstrate a 45,X karyotype, confirming the diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it suggests a different genetic disorder, such as Klinefelter syndrome, which is characterized by an extra X chromosome (47,XXY) and typically presents with tall stature and infertility in males.

**Option B:** This option is incorrect because it suggests a normal karyotype, which would not explain the girl's clinical presentation of short stature, neck webbing, and sexual infantilism.

**Option C:** This option is incorrect because it suggests a different genetic disorder, such as Down syndrome, which is caused by an extra copy of chromosome 21 (trisomy 21) and presents with distinct clinical features such as intellectual disability and characteristic facial features.

**Clinical Pearl / High-Yield Fact**
Turner syndrome is a classic example of a chromosomal abnormality that can present with a wide range of physical and developmental abnormalities. It is essential to consider this diagnosis in any female patient with short stature, neck webbing, and gonadal dysgenesis.

**Correct Answer:** D. 45,X karyotype