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Down syndrome is characterized by the presence of an extra chromosome, specifically a third copy of chromosome 21. This condition, also known as trisomy 21, results from an error in cell division during the formation of the egg or sperm, leading to the individual having three copies of chromosome 21 instead of the usual two. This additional genetic material affects development and can lead to a range of physical and intellectual disabilities.

The other conditions listed do not result from the presence of an extra chromosome. Turner Syndrome involves the complete or partial absence of one X chromosome in females, leading to a total of 45 chromosomes rather than the typical 46. Fragile X Syndrome is caused by a mutation in the FMR1 gene on the X chromosome but does not involve an extra chromosome. Cystic Fibrosis is a genetic disorder caused by mutations in the CFTR gene, which also does not involve an additional chromosome. Therefore, Down syndrome is uniquely defined by having an extra chromosome, making it the correct choice in this context.