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NBME 16 Answers

nbme16/Block 4/Question#48 (16.7 difficulty score)
A 32-year-old woman, gravida 4, para 0, ...
Unbalanced chromosome rearrangementπŸ”
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 +3 
submitted by feliperamirez(31),
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bingcentipede  "Unbalanced chromosome rearrangement" is also a description of trisomy in and of itself +  
bingcentipede  "Unbalanced chromosome rearrangement" is also a description of trisomy in and of itself +  



 +2 
submitted by utap2001(4),

There are three types of Down syndrome: trisomy 21 (nondisjunction), translocation and mosaicism.

Trisomy 21, the most common type of Down syndrome, occurs when there are three, rather than two, number 21 chromosomes present in every cell of the body. Instead of the usual 46 chromosomes, a person with Down syndrome has 47. It is this additional genetic material that alters the course of development and causes the characteristics associated with the syndrome. Trisomy 21 accounts for 95% of cases.

Translocation accounts for 4% of all cases of Down syndrome. In translocation, part of chromosome 21 breaks off during cell division and attaches to another chromosome, typically chromosome 14. While the total number of chromosomes in the cells remain 46, the presence of an extra part of chromosome 21 causes the characteristics of Down syndrome.

Mosaicism occurs when nondisjunction of chromosome 21 takes place in one – but not all – of the initial cell divisions after fertilization.When this occurs, there is a mixture of two types of cells, some containing the usual 46 chromosomes and others containing 47. Mosaicism accounts for about 1% of all cases of Down syndrome.

Uworld genetic section has similar question.




 +2 
submitted by notyasupreme(18),

I was thinking it could be 22q11 microdeletion because that also would have abnormal organs (heart, thymus, parathyroids) and abnormal face. So, I'm just wondering how you're supposed to definitely know it was Trisomy 21. I had the original answer too, damnit!

nbmeanswersownersucks  I think it's more because a spontaneous microdeletion would not really explain the previous lost pregnancies whereas an unbalanced translocation in the mom could. And I think it could be any of the big trisomies (13, 18, 21) but 21 is most common. +3  
m0niagui  so, it is the mother that has Down's Sd? +  
jj375  No, the mother doesnt have down syndrome. As far as I know, the mother has a BALANCED translocation - for example (totally made the chromosome # up) she may have a piece of chrom 11 and chrom 4 swap places. However they are balanced since they are both there, no genetic material is missing os she looks healthy and is fine. BUT when her eggs are made, some eggs may get the chrom 11 (+ piece of chrom 4) and then her regular copy of chrom 4...Now that baby also gets a chromosome 4 from the dad. The baby will end up with 3 copies of part of chrom 4 but 1 part of chrom 11! So the kid is unbalanced and may die in utero (spontaneous abortions). I hope that was clear Here is a photo that may help: https://accessmedicine.mhmedical.com/data/books/hoff2/hoff2_c006f005.png +