Which of the following is an example of a chromosomal abnorm?

Fragile X syndrome is an example of a chromosomal abnormality because it is caused by a mutation in the FMR1 gene located on the X chromosome. Specifically, this condition is characterized by the expansion of a CGG trinucleotide repeat within this gene, leading to a range of cognitive and behavioral issues. The presence of the extra repeats disrupts normal gene function, demonstrating how structural changes in chromosomes can result in developmental disorders.

In contrast, the other options relate to different types of genetic or developmental issues. Neural tube defects are typically due to environmental factors and nutritional deficiencies, particularly related to folic acid, rather than chromosomal abnormalities. Cystic fibrosis, while genetic, is the result of mutations in a single gene (CFTR) rather than a chromosomal alteration. Finally, breast cancer associated with a BRCA mutation, although linked to genetic predisposition, is also the result of specific gene alterations rather than chromosomal abnormalities. Therefore, Fragile X syndrome stands out as the clear example of a chromosomal abnormality among the choices given.