The INCORRECT Statement is E: Prenatal Diagnosis is Not Possible
Statement E is definitively incorrect—prenatal diagnosis for Fragile X syndrome is absolutely possible and well-established through both amniocentesis and chorionic villus sampling (CVS). 1
Analysis of Each Statement
Statement A: Second Most Common Cause of Intellectual Disability in Males After Down Syndrome
CORRECT. Fragile X syndrome is indeed the second most common cause of intellectual disability in males after Down syndrome, and is the most common inherited cause of intellectual disability 2, 3. The prevalence is estimated at approximately 1 in 4,000 males 1.
Statement B: Less Frequent and Milder Phenotype in Females
CORRECT. The female prevalence is approximately half that of males (1 in 5,000-8,000) 1. Critically, less than 50% of females with full mutations have intellectual disability 1. This milder phenotype results from nonrandom X-inactivation, though females may still manifest symptoms including avoidance personality, mood disorders, or stereotypic behaviors 1.
Statement C: Increased (>200) Trinucleotide Repeats in FMR1 Gene
CORRECT. Full mutations are defined by >200 CGG repeats in the FMR1 gene 4, 5, 6. This expansion induces DNA methylation of the promoter region, leading to transcriptional silencing and loss of FMRP protein, which is the molecular basis of the syndrome 4, 6.
Statement D: Small Gap or Fragile Site on Long Arm of X Chromosome
CORRECT. The syndrome is named for the characteristic fragile site visible on cytogenetic analysis of the long arm of the X chromosome, though modern molecular testing has largely replaced this historical diagnostic method.
Statement E: Prenatal Diagnosis is Not Possible
INCORRECT. The ACMG guidelines explicitly state that prenatal testing can be performed using cells obtained from both amniocentesis and CVS 1. The guidelines detail specific technical considerations:
- CVS testing is feasible but requires awareness that methylation associated with lyonization may not be fully established at the time of sampling 1
- Amniocentesis is fully validated for prenatal diagnosis
- It is acceptable to omit methylation analysis entirely when testing CVS specimens and rely on repeat size alone 1
- In ambiguous CVS cases, follow-up amniocentesis may be required 1
Clinical Implications
The availability of prenatal diagnosis is critical for genetic counseling in families with known Fragile X mutations. The ACMG and American Congress of Obstetricians and Gynecologists have published specific recommendations for carrier detection and prenatal testing 1. Early diagnosis through prenatal screening is particularly important given that there is no cure for the condition, and management relies on early intervention with behavioral, physical, occupational, and speech-language therapies 2.