**Core Concept**
Down syndrome is a genetic disorder caused by an extra copy of chromosome 21, leading to various physical and cognitive abnormalities. It is characterized by distinct clinical features, including intellectual disability, delayed speech, and characteristic facial features.

**Why the Correct Answer is Right**
The correct answer will not be a feature of Down syndrome. The features of Down syndrome include intellectual disability, delayed speech, characteristic facial features, obesity, short stature, and increased risk of congenital heart defects. These features arise from the altered gene expression and protein production resulting from the extra copy of chromosome 21.

**Why Each Wrong Option is Incorrect**
**Option A:** Intellectual disability is a hallmark of Down syndrome, resulting from the altered gene expression and protein production. This is a direct consequence of the extra copy of chromosome 21.
**Option B:** Delayed speech is a common feature of Down syndrome, often due to the associated intellectual disability and developmental delays.
**Option C:** Characteristic facial features, such as a flat face, short nose, and protruding tongue, are typical of Down syndrome.

**Clinical Pearl / High-Yield Fact**
The risk of congenital heart defects in Down syndrome is significantly higher than in the general population, with up to 50% of individuals having some form of heart defect.

**Correct Answer:** D.