**Core Concept**
Patau's syndrome is a genetic disorder caused by an extra copy of chromosome 13. This condition leads to severe intellectual disability, physical abnormalities, and a high risk of mortality. The extra copy of chromosome 13 disrupts normal gene expression, resulting in the characteristic features of Patau's syndrome.

**Why the Correct Answer is Right**
Trisomy 13 occurs when there is an extra copy of chromosome 13, leading to an abnormal number of genes being expressed. This can result in various physical and developmental abnormalities, including microcephaly, cleft palate, and holoprosencephaly. The extra genetic material on chromosome 13 disrupts normal brain development and function, leading to severe intellectual disability.

**Why Each Wrong Option is Incorrect**
* **Option A:** Trisomy 21 causes Down syndrome, characterized by intellectual disability, short stature, and distinctive facial features. It is not associated with Patau's syndrome.
* **Option B:** Trisomy 18, also known as Edwards syndrome, is a different genetic disorder that leads to severe intellectual disability, heart defects, and physical abnormalities. It is not the cause of Patau's syndrome.
* **Option C:** 18 P refers to a translocation involving chromosome 18, which is not the cause of Patau's syndrome.

**Clinical Pearl / High-Yield Fact**
Patau's syndrome is often associated with a characteristic "triple bubble" sign on ultrasound, indicating three separate fluid-filled structures in the abdominal cavity.

**✓ Correct Answer:** D. Trisomy 13