Which of the following is NOT considered a single-gene disorder?

Edwards syndrome, also known as trisomy 18, is a chromosomal disorder caused by the presence of an extra 18th chromosome. This condition affects multiple systems in the body and is associated with a range of physical and developmental anomalies. Unlike single-gene disorders, which result from mutations in a specific gene and typically follow Mendelian inheritance patterns, chromosomal disorders involve changes in chromosomal structure or number, impacting overall genetic material.

In contrast, conditions such as cystic fibrosis, phenylketonuria, and Huntington's disease are all examples of single-gene disorders. Cystic fibrosis stems from a mutation in the CFTR gene, phenylketonuria results from mutations in the PAH gene, and Huntington's disease is linked to mutations in the HTT gene. Each of these disorders is inherited in a specific manner, either autosomal recessive or autosomal dominant, which defines them as single-gene disorders. Therefore, Edwards syndrome stands apart due to its chromosomal basis rather than being the result of a specific gene mutation.