<prompt> A 22-year-old man complains about his inability to conceive a child. On physical examination, the patient is noted to be tall 6 ft, 5 in and exhibits gynecomastia and testicular atrophy. Laboratory studies demonstrate increased serum levels of follicle-stimulating hormone. Cytogenetic studies reveal a chromosomal abnormality. What is the most common cause of this patients chromosomal abnormality?Expansion of a trinucleotide repeatIsochromosome formationMeiotic nondisjunctionNonreciprocal translocation  <response>: Klinefelter syndrome, or testicular dysgenesis, is related to the presence of one or more X chromosomes in excess of the normal male XY complement. Most persons with Klinefelter syndrome 80 have one extra X chromosome 47, XXY karyotype. The additional X chromosomes arises as a result of nondisjunction during gametogenesis. In half of cases, nondisjunction occurs during paternal meiosis I, leading to a sperm containing both an X and a Y chromosome. Fertilization of a normal oocyte by such a sperm gives a zygote with a 47, XXY complement of chromosomes. Klinefelter syndrome occurs in 1 per 1,000 male newborns, which is roughly comparable to the incidence of Down syndrome. None of the other choices are associated with trisomy.Diagnosis: Klinefelter syndrome 