During what stage does nondisjunction most commonly occur in human oocyte development?

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division, which can lead to chromosomal abnormalities. In the context of oocyte development, nondisjunction most frequently occurs during the first meiotic division, specifically at Metaphase I.

During Metaphase I, homologous chromosomes are aligned at the metaphase plate and are supposed to separate to opposite poles. If they do not separate correctly during this stage, it can result in an oocyte with an abnormal number of chromosomes. This is particularly significant because oocytes are formed during fetal development and can remain arrested in prophase I until ovulation, making the long duration of this arrest a factor in the increased risk of nondisjunction.

Once ovulation occurs and meiosis resumes, the oocyte completes its first meiotic division, transitioning from Metaphase I to Anaphase I. If nondisjunction occurs at that critical stage, it can lead to conditions such as Down syndrome or other aneuploidies in the resulting embryo.

Therefore, the correct answer is that nondisjunction most commonly occurs during Metaphase I of oocyte development.