**Core Concept**
Patau's syndrome is a genetic disorder caused by an extra copy of chromosome 13, leading to severe intellectual disability, physical abnormalities, and increased risk of early death. This condition is a result of trisomy 13, a chromosomal abnormality where an individual has three copies of chromosome 13 instead of the usual two.

**Why the Correct Answer is Right**
Trisomy 13 occurs when there is a failure of proper chromosome separation during meiosis, resulting in an extra copy of chromosome 13. This extra genetic material disrupts the normal development and function of various organs and systems in the body, leading to the characteristic features of Patau's syndrome. The affected individuals often have severe physical abnormalities, including microcephaly, cleft palate, and polydactyly.

**Why Each Wrong Option is Incorrect**
**Option A:** Trisomy 21, also known as Down syndrome, is a different chromosomal abnormality that results from an extra copy of chromosome 21, not chromosome 13.

**Option B:** Trisomy 18, also known as Edwards syndrome, is another chromosomal abnormality that results from an extra copy of chromosome 18, not chromosome 13.

**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 a rare chromosomal disorder with a high mortality rate, often due to congenital heart defects, respiratory problems, and seizures. It's essential to recognize the characteristic features of this condition to provide accurate diagnosis and management.

**Correct Answer:**
✓ Correct Answer: D. Trisomy 13