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Fragile X Syndrome 1

Fragile X Syndrome


Vignette: A 10-year-old boy is brought to the pediatric clinic by his mother for a regular checkup. His mother expresses concern about her son's height as he is shorter than his peers. She also mentions that her son has been struggling in school, especially with reading and writing. On physical examination, the boy has a long face with a prominent forehead, large ears, and macroorchidism. Genetic testing is performed and confirms a diagnosis. Which of the following is the most likely mechanism of the genetic mutation in this patient?


A. Deletion of a gene

B. Duplication of a gene

C. Expansion of trinucleotide repeats

D. Missense mutation

E. Nonsense mutation


C. Expansion of trinucleotide repeats


The patient's presentation of cognitive impairment, characteristic facial features, and macroorchidism suggests Fragile X syndrome, a condition that is caused by an expansion of trinucleotide repeats (specifically CGG repeats) in the FMR1 gene located on the X chromosome. This gene is responsible for producing a protein that is important for nerve development. If the CGG segment repeats more than 200 times, the FMR1 gene is turned off, or silenced, and the necessary protein is not made. This is the most common form of inherited intellectual disability in males. Choice A, B, D, and E are other mechanisms of genetic mutations, but they are not associated with Fragile X syndrome.


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