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Retired NBME Free 120 Answers

free120/Block 1/Question#33 (reveal difficulty score)
A 2-year-old boy is brought to the physician ...
Trinucleotide repeat expansion 🔍 / 📺 / 🌳 / 📖
tags: biochem genetics repeat

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submitted by bwdc(697)
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Fragile X is a CGG trinucleotide repeat expansion disorder (which like Huntington’s is a test favorite). The maternal uncle is the hint to the X-linked inheritance. Autism-like behaviors and relatively large head are common; large testicles only appear after puberty.

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cbreland  Also repeat expansions will get worse with each generation. I went with the learning difficulties and then uncle having issues that were not as severe to a child with issues within the first 2 years of life as being a signal of a triplet expansion +1
osteopathnproud  Fragile X syndrome is the most common inherited cause of intellectual disability FA20 pg 62. I just saw intellectual disability plus heritable = fragile X syndrome. +2


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submitted by drdoom(1206)
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This is a corrected version of the post from @abhishek021196 below (https://nbmeanswers.com/exam/free120/1426#3919).


Fragile X syndrome X-linked dominant inheritance. Trinucleotide repeat in FMR1 gene = hypermethylation = decreased expression.

Most common inherited cause of intellectual disability (Down syndrome is the most common genetic cause, but most cases occur sporadically).

Findings: post-pubertal macroorchidism (enlarged testes), long face with a large jaw, large everted ears, autism, mitral valve prolapse, hypermobile joints.

Trinucleotide repeat expansion [(CGG) n ] occurs during oogenesis.

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submitted by amy(4)
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Learned something new today. " hand flapping" associates not only with hepatic encephalopathy, but also autism behavior.....

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drdoom  hand flapping ≠ asterixis +2
drdoom  hand flapping -> https://youtu.be/4ALy6I1J1uo +2
drdoom  another asterixis (notice how holding her hand in extension causes a contralateral clonus) -> https://youtu.be/-R3lRmu7dbk +1


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submitted by lukin4answer(11)
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Can anyone clarify why "BOTH PARENTS had learning difficulties??""

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freemanpeng  Because they are perfect match for each other i believe +1


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submitted by namesthegame22(13)
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Anticipation is a genetic phenomenon in which disease traits are more severe and/or appear at earlier ages in later generations. Anticipation typically results from trinucleotide repeat expansions. Myotonic dystrophy, which features intergenerational anticipation, results from a trinucleotide repeat expansion of the DMPK gene in skeletal muscle cells that leads to progressive muscle dysfunction.

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submitted by abhishek021196(119)
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Fragile X syndrome X-linked dominant inheritance. Trinucleotide repeat in FMR1 gene = hypermethylation = Increased expression.

Most common inherited cause of intellectual disability (Down syndrome is the most common genetic cause, but most cases occur sporadically).

Findings: post-pubertal macroorchidism (enlarged testes), long face with a large jaw, large everted ears, autism, mitral valve prolapse, hypermobile joints.

Trinucleotide repeat expansion [(CGG) n ] occurs during oogenesis.

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melanoma  decreased expression +10
jbrito718  one mistake and this guys great explanation plummets. Bunch of savages +3

decreased expression. Sorry for the mistake. I don't have an edit option.


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