This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: NEET 2019, Subject name: Pediatrics
True about Fragile X syndrome is- 
Select and explain. 
A. Triple nucleotide CAG Sequence mutation, B. 10% Female carriers mentally retarded, C. Males have IQ 20-40, D. Gain of function mutation
B: 10% Female carriers mentally retarded
Answer: B: 10% Female carriers mentally retarded 
Explanation: Solutions: Majority of female carriers of FRAGILE X SYNDROME are mentally normal, 1 in 3 female carriers have some evidence of mental impairment and about 1 in 10 are considered mentally retarded I.e. 10% females are mentally retarded. Mean IQ in males with full mutation (>200 repeats) is 42 Fragile X syndrome Trinucleotide repeat mutation in familial mental retardation -1 (FMR1) gene Second most common genetic cause of mental retardation after Down Syndrome Genetic basis Normal population : 29-55 CGG repeats Carriers: pre-mutatins 55-200 CGG repeats Full mutation (Fragile X syndr): 4000 repeats