Question 142

Question:

A mother brings her two year-old son to your office because he has not yet spoken any words. On exam, he is very quiet, has a prominent jaw, large ears and large testicles. His mother states that she has a brother who is mentally retarded. Cytogenetic analysis reveals a genetic defect to be the cause of the patient’s mental retardation. What is the mostly likely mechanism of this genetic defect?

Correct Answer: D.  Trinucleotide repeat

Fragile X syndrome is the most common cause of mental retardation in males. Key diagnostic factors are lack of milestone attainment, especially language, from a young age, large ears, large testes, a prominent jaw, a long face and flat feet. It is X-linked recessive inheritance that is caused by increased number of CGG repeats on the FMR1 gene. Normal CGG repeats are between 6 and 55, while Fragile X FMR1 alleles have over 230 repeats.