A 3-year-old boy is brought to the office for follow-up on intellectual disability and speech delay. Review of his developmental screening questionnaire shows that the boy does not imitate his parents' activities, have a social smile, or show interest in other children. The patient has 8-10 words in his vocabulary and does not combine them into 2-word phrases. There is also concern that his verbal comprehension is poor, and a subsequent audiological evaluation was normal. The boy is up to date on all vaccinations and has no other significant medical history. Laboratory testing shows 226 CGG trinucleotide repeats in a gene located on the X chromosome. Which of the following is the most likely cause of this patient's clinical condition?

Question

A 3-year-old boy is brought to the office for follow-up on intellectual disability and speech delay.  Review of his developmental screening questionnaire shows that the boy does not imitate his parents' activities, have a social smile, or show interest in other children.  The patient has 8-10 words in his vocabulary and does not combine them into 2-word phrases.  There is also concern that his verbal comprehension is poor, and a subsequent audiological evaluation was normal.  The boy is up to date on all vaccinations and has no other significant medical history.  Laboratory testing shows 226 CGG trinucleotide repeats in a gene located on the X chromosome.  Which of the following is the most likely cause of this patient's clinical condition?

Quizplus · Accepted Answer

Eductional Objective: Fragile X syndrome is caused by an increased number of CGG trinucleotide repeats on the fragile X mental retardation 1 (FMR1) gene on the long arm of the X chromosome.  This leads to hypermethylation and inactivation of FMR1.

A 3-Year-Old Boy Is Brought to the Office for Follow-Up