**Core Concept**
Down syndrome is a chromosomal disorder caused by the presence of an extra copy of chromosome 21 (trisomy 21). It is the most common chromosomal syndrome, characterized by intellectual disability, distinctive physical features, and increased risk of certain health problems.

**Why the Correct Answer is Right**
Down syndrome occurs due to nondisjunction during meiosis, resulting in an extra copy of chromosome 21. This extra genetic material disrupts normal cellular function, leading to the characteristic symptoms and complications associated with the condition. The risk of Down syndrome increases with advanced maternal age, particularly after the age of 35.

**Why Each Wrong Option is Incorrect**
**Option A:** Other chromosomal syndromes, such as Turner syndrome and Klinefelter syndrome, are less common than Down syndrome.
**Option B:** Fragile X syndrome is a genetic disorder caused by a mutation on the X chromosome, not a chromosomal duplication like Down syndrome.
**Option C:** Edwards syndrome is a chromosomal disorder caused by trisomy 18, which is less common than Down syndrome.

**Clinical Pearl / High-Yield Fact**
Down syndrome is associated with an increased risk of congenital heart defects, particularly atrioventricular septal defects. Patients with Down syndrome are also at higher risk for leukemia and Alzheimer's disease.

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