**Core Concept**
Prader-Willi syndrome (PWS) is a rare genetic disorder characterized by a deletion or maternal disomy (two copies of chromosome 15 from the mother) of the paternal copy of chromosome 15. This condition is caused by the absence of genes on the paternal chromosome 15, which are crucial for normal growth and development.

**Why the Correct Answer is Right**
Maternal disomy of chromosome 15 occurs when a person inherits two copies of chromosome 15 from their mother, resulting in the absence of the paternal copy. This leads to the deletion of genes that are normally present on the paternal chromosome 15. In PWS, the deletion of these genes on chromosome 15 leads to a range of symptoms, including short stature, intellectual disability, and a characteristic facial appearance. The absence of the paternal copy of chromosome 15 disrupts the normal expression of genes involved in growth and development, resulting in the characteristic features of PWS.

**Why Each Wrong Option is Incorrect**
**Option B:** Klinefelter's syndrome is a genetic condition in males characterized by an extra X chromosome (XXY), which is not related to maternal disomy of chromosome 15.
**Option C:** Angelman syndrome is a genetic disorder caused by a deletion or mutation of the UBE3A gene on chromosome 15, but it is typically associated with paternal deletion of chromosome 15, not maternal disomy.
**Option D:** Turner's syndrome is a genetic condition in females characterized by the absence of one X chromosome (XO), which is not related to maternal disomy of chromosome 15.

**Clinical Pearl / High-Yield Fact**
It's essential to note that Prader-Willi syndrome can also be caused by a paternal deletion of chromosome 15, not just maternal disomy. Understanding the genetic basis of PWS is crucial for diagnosis and management.

**✓ Correct Answer: A. Prader - Willi syndrome**