## Core Concept
Down's syndrome, also known as Trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. This condition leads to intellectual disability, delayed speech, and distinct physical characteristics. The core concept here revolves around the genetic and clinical features associated with Down's syndrome.

## Why the Correct Answer is Right
The correct answer, which is not provided in the query, typically relates to a statement that does not accurately describe a characteristic or aspect of Down's syndrome. Generally, statements about Down's syndrome might include its genetic cause (trisomy 21), common physical features (e.g., flat face, short neck), developmental delays, and increased risk of certain medical conditions (e.g., heart defects). The accuracy of the statement regarding Down's syndrome would depend on its specifics, which are not provided.

## Why Each Wrong Option is Incorrect
- **Option A:** Without the specific details of option A, it's challenging to explain why it's incorrect. However, if option A stated a true characteristic of Down's syndrome, such as an increased risk of congenital heart defects or intellectual disability, it would not be the correct answer to the question of what is "not true."
- **Option B:** Similarly, if option B presented a known feature or association of Down's syndrome, it would be incorrect as an answer to what is not true about the condition.
- **Option C:** This option would also be incorrect if it accurately described a common aspect of Down's syndrome.
- **Option D:** If option D is the correct answer, then options A, B, and C must all present true statements about Down's syndrome, making them incorrect choices for what is not true.

## Clinical Pearl / High-Yield Fact
A key point to remember about Down's syndrome is that individuals with this condition are at an increased risk for certain medical conditions, such as congenital heart defects, Alzheimer's disease, and leukemia. Early screening and intervention can significantly improve outcomes for these patients.

## Correct Answer: D. Increased longevity.