Which condition is characterized by an extra copy of chromosome 21 and is associated with mild to moderate intellectual disability and physical features?

• A. Klinefelter syndrome
• B. Turner syndrome
• C. Fragile X syndrome
• D. Down syndrome

Answer: (D)

An extra copy of chromosome 21 causes trisomy 21, which is Down syndrome. This condition is commonly associated with mild to moderate intellectual disability and distinctive physical features such as a flattened facial profile, upward-slanting eyes, small ears, and sometimes reduced muscle tone. It can also include health issues like congenital heart defects. The other options involve different chromosomal changes or gene mutations: Klinefelter syndrome is XXY in males, Turner syndrome is monosomy X in females, and Fragile X syndrome is a mutation on the X chromosome (not an extra chromosome). This combination of an extra chromosome 21 with the described cognitive and physical traits best matches Down syndrome.