All are true about Fragile X syndrome except ?
Fragile X is the most common inherited cause of intellectual disability and autism. It's an X-linked dominant disorder caused by a CGG trinucleotide repeat expansion in the FMR1 gene. The full mutation leads to methylation and silencing of the gene, resulting in absence of FMRP protein. Common features include macroorchidism, elongated face, and cognitive impairment. Males are more severely affected than females.
The question asks which statement is incorrect. Let's think about typical incorrect options. For example, if an option says it's autosomal recessive, that's wrong. Another might incorrectly state it's caused by a deletion instead of a repeat expansion. Also, maybe a wrong option says females are more affected, which is incorrect because males are. Another possible wrong fact is that it's the second most common cause after Down syndrome, but actually, Down is different and Fragile X is the leading genetic cause.
Now, the correct answer in the given example is missing, but assuming the options include one of these incorrect statements. Let's say option B says it's autosomal recessive. Then, the correct answer would be B. The explanation would detail that it's X-linked, not autosomal. Each wrong option needs a brief explanation. The clinical pearl should highlight the X-linked nature and the CGG repeats.
Wait, the user provided the correct answer as "D. [Answer Text]" but in the example, the correct answer line is missing. Maybe in the actual question, the correct answer is D, but the user's example shows that the correct answer is missing. Hmm, perhaps the user made a mistake in the example. But since the task is to create the explanation based on the given correct answer, even if it's incomplete, I need to proceed with the structure.
Assuming the correct answer is D, but the actual correct answer is different. Let me recheck. The correct answer for the question "All are true except..." would be the option that states a false statement about Fragile X. For example, if one option says it's caused by a deletion, that's wrong. So, the correct answer would be that option.
Putting it all together: Core concept is Fragile X's genetics and inheritance. Correct answer is the one that incorrectly states the inheritance pattern. Wrong options are explained based on common misconceptions. The clinical pearl is about the CGG repeats and X-linked transmission.
**Core Concept**
Fragile X syndrome is the most common inherited cause of intellectual disability and autism spectrum disorder, caused by a trinucleotide (CGG) repeat expansion in the *FMR1* gene on the X chromosome. It follows an X-linked dominant pattern with variable expressivity and anticipation.
**Why the Correct Answer is Right**
The incorrect statement in the question likely pertains to a misconception about inheritance or clinical features. For example, if an option claims it is autosomal recessive, this is false because Fragile X is X-linked dominant. The CGG repeat expansion leads to methylation and silencing of the *FMR1* gene, resulting in absence of the fragile X mental retardation protein