What is the pathophysiological cause of Down syndrome?

The pathophysiological cause of Down syndrome is primarily attributed to nondisjunction during meiosis, specifically involving chromosome 21. Nondisjunction refers to the failure of homologous chromosomes or sister chromatids to separate properly during cell division. When this error occurs during meiosis I, it results in gametes that either have an extra copy of chromosome 21 or are missing one.

When a gamete with an extra chromosome 21 combines with a normal gamete during fertilization, the resulting zygote has three copies of chromosome 21, leading to the condition known as trisomy 21, which is commonly associated with Down syndrome. This accounts for the primary incidence of the disorder, as approximately 95% of cases are the result of this nondisjunction event.

Other options do not correctly represent the primary cause of Down syndrome. Chromosomal inversion does not lead to the number of chromosomes changing in a way that would result in Down syndrome, and translocation of chromosome 14 refers to a different chromosomal abnormality. A deficiency in chromosome 21 does not occur, as Down syndrome is characterized by the presence of an extra chromosome rather than a loss. Thus, the correct answer emphasizes the significance of nondisjunction and its direct