Which condition is caused by an extra 21st chromosome (Trisomy 21)?

Down's syndrome, also known as Trisomy 21, is characterized by the presence of an extra chromosome 21. This genetic condition arises during the formation of reproductive cells, where an error in cell division results in the embryo having three copies of chromosome 21 instead of the usual two.

Individuals with Down's syndrome often exhibit distinct physical features, such as a flat facial profile, slanted eyes, and a protruding tongue, as well as experiencing varying degrees of intellectual disability and potential health issues, including congenital heart defects. The presence of this extra chromosome significantly impacts physical and cognitive development.

Turner's syndrome and Klinefelter's syndrome are related to abnormalities in sex chromosomes, not chromosome 21. Turner's syndrome is characterized by the absence of one X chromosome in females, while Klinefelter's syndrome involves an extra X chromosome in males. Astigmatism is a refractive error of the eye caused by an irregular curvature of the cornea or lens, which is unrelated to chromosomal abnormalities.

Thus, Down's syndrome is specifically linked to the extra 21st chromosome, making it the correct answer to the question.