**Core Concept**
Prader-Willi syndrome (PWS) is a rare genetic disorder characterized by a spectrum of physical, mental, and behavioral features. The syndrome is caused by a loss of function of genes located on chromosome 15, specifically the paternally inherited region.

**Why the Correct Answer is Right**
The correct answer is related to the deletion or uniparental disomy of the paternal copy of chromosome 15. This results in the silencing of genes on the paternal chromosome 15, leading to the characteristic features of PWS. The paternally expressed genes on chromosome 15 are crucial for normal growth and development, and their loss leads to the clinical manifestations of PWS.

**Why Each Wrong Option is Incorrect**
**Option A:** Deletion of the maternal copy of chromosome 15. This would lead to Angelman syndrome, a different genetic disorder characterized by developmental delays, seizures, and ataxia.

**Option B:** Trisomy 15. This would result in a different set of clinical features and is not associated with PWS.

**Option C:** Mutation in the PAX6 gene. This gene is involved in eye development and mutations in PAX6 are associated with aniridia and other eye abnormalities, not PWS.

**Option D:** Deletion of chromosome 18. Deletions of chromosome 18 are associated with various clinical features, including intellectual disability, but are not characteristic of PWS.

**Clinical Pearl / High-Yield Fact**
Prader-Willi syndrome is characterized by a distinctive facial appearance, short stature, hypogonadism, and intellectual disability, as well as characteristic behavioral features such as hyperphagia and obsessive-compulsive disorder.

**Correct Answer:** D. Deletion of chromosome 18.