Which of the following is one of the two most common genetic causes of intellectual disabilities?

Down syndrome is recognized as one of the two most common genetic causes of intellectual disabilities due to its prevalence and the specific chromosomal abnormality associated with it. It is caused by the presence of an extra copy of chromosome 21, resulting in a triplication known as trisomy 21. This additional genetic material affects normal cognitive and physical development, leading to a spectrum of intellectual disabilities and various health issues.

Down syndrome is widely studied and documented, making it a key focus in special education and genetic counseling. Its clear association with a set of characteristics and developmental delays is an important factor in identifying and providing appropriate supports for individuals with intellectual disabilities.

In contrast, the other options listed do not primarily cause intellectual disabilities. Acanthosis nigricans is a skin condition often related to insulin resistance, attention-deficit/hyperactivity disorder (ADHD) is primarily a behavioral disorder and not a genetic cause of intellectual disabilities, and osteogenesis imperfecta primarily affects bone density and structure rather than directly causing intellectual disabilities. These distinctions underscore why Down syndrome stands out as a significant genetic factor contributing to intellectual disabilities.