## **Core Concept**
Trisomy 13, also known as **Patau syndrome**, is a genetic disorder caused by an extra copy of chromosome 13. This condition leads to severe intellectual disability and physical abnormalities. It is one of the most common chromosomal abnormalities.

## **Why the Correct Answer is Right**
Trisomy 13 occurs when there is an extra copy of chromosome 13, making a total of three copies instead of the usual two. This extra genetic material disrupts the normal development of the fetus, leading to the characteristic features of Patau syndrome. These features include severe intellectual disability, heart defects, brain or spinal cord abnormalities, very small or poorly developed eyes (microphthalmia), extra fingers or toes, cleft lip with or without cleft palate, low-set ears, and weak muscle tone.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not accurately describe Trisomy 13.
- **Option B:** This option is incorrect because it refers to a different condition.
- **Option C:** This option is incorrect as it does not correspond with the definition of Trisomy 13.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl for Trisomy 13 (Patau syndrome) is that it has a very poor prognosis, with most individuals not surviving beyond the first year of life due to severe medical complications. Diagnostic features such as microcephaly, polydactyly, and severe intellectual disability can aid in clinical suspicion, which is then confirmed by chromosomal analysis.

## **Correct Answer:** D. Patau syndrome.