**Core Concept**
The most common chromosomal syndrome refers to the most frequently occurring genetic disorder caused by an abnormal number of chromosomes or structural abnormalities in a chromosome. This condition affects various aspects of an individual's development and physical characteristics.

**Why the Correct Answer is Right**
The most common chromosomal syndrome is Down syndrome, which is caused by the presence of an extra copy of chromosome 21 (trisomy 21). This results in an individual having 47 chromosomes instead of the typical 46. The extra genetic material leads to the development of characteristic physical features, intellectual disability, and various medical complications. The presence of an extra chromosome 21 leads to the overexpression of genes involved in cell growth and division, which disrupts normal cellular processes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify a known chromosomal syndrome. Without more information, it is difficult to determine which chromosomal disorder this option refers to.
**Option B:** This option is incorrect as it does not accurately represent the most common chromosomal syndrome. While Turner syndrome is a significant chromosomal disorder, it is not the most common.
**Option C:** This option is incorrect as it is not a recognized chromosomal syndrome. Without further information, it is difficult to determine what this option refers to.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Down syndrome is caused by an extra copy of chromosome 21, which leads to the overexpression of genes involved in cell growth and division. This results in characteristic physical features and intellectual disability.

**Correct Answer: D. Down syndrome.**