**Core Concept**
Prader Willi syndrome (PWS) is a genetic disorder caused by the loss of function of genes on chromosome 15 inherited from the father. Maternal disomy occurs when a person inherits two copies of chromosome 15 from their mother, resulting in the absence of paternal genes.

**Why the Correct Answer is Right**
Maternal disomy of chromosome 15 is a rare event that disrupts the normal balance of genes on this chromosome. In the case of PWS, the absence of paternal genes leads to the silencing of the paternally expressed genes, including the SNRPN gene, which is crucial for normal development. This disruption causes the characteristic features of PWS, including short stature, intellectual disability, and hyperphagia.

**Why Each Wrong Option is Incorrect**
**Option A is incorrect because**: While Klinefelter syndrome is indeed a sex chromosome disorder, it is caused by an extra X chromosome (XXY), not maternal disomy of chromosome 15.

**Option B is incorrect because**: Angelman syndrome is typically caused by a deletion or mutation of the UBE3A gene on chromosome 15, inherited from the mother. Maternal disomy of chromosome 15 is not a common cause of Angelman syndrome.

**Option C is incorrect because**: A hydatidiform mole is a type of gestational trophoblastic disease, characterized by an abnormal proliferation of trophoblastic cells, but it is not directly related to maternal disomy of chromosome 15.

**Clinical Pearl / High-Yield Fact**
Key to differentiating PWS from Angelman syndrome is the presence of a normal karyotype (46, XX or XY) in PWS patients, as opposed to the typical deletion or mutation of the UBE3A gene in Angelman syndrome.

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