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NBME 21 Answers

nbme21/Block 4/Question#13

A 4-month-old boy is diagnosed with a rare ...

Dilated rough endoplasmic reticulum

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submitted by sklawpirt(19),

I think the idea here is simply that one should think about where vesicles are coming from on their way to the golgi complex.

"Two steps forward and one step back." Specfically the question may be referring to a rare craniofacial disorder. an awarenesss of that disease is not necessary. What is necessary is understanding the origin from where vesicles are traficked to the Golgi apparatus.

COPI protein is needed to coat vescles from the RER to send to golgi. Thus, with a mutation in that protein, the packaged proteins that should bleb off and be sent to the golgi, instead accumulate in the RER and dilate it. Thus the answer.

https://www.cell.com/ajhg/pdf/S0002-9297(16)30214-2.pdf

hayayah  pg. 47 on FA got the good visuals! +2  
notadoctor  COPII* proteins are needed to coat vesicles from the RER to Golgi. "Two(COPII) steps forward; one(COPI) step back." Anterograde goes RER -> Golgi -> Lysosomes/Secretory Vesicles -> Plasma membrane +13  
titanesxvi  why not small lysosomes? +2  
varunmehru  and I thought large lysosomes due to lack of enzymes to degrade +  
samsam3711  The size of the lysosome is not affected by the presence or absence of protein, but its function is compromised (eg. protein is getting stuck in the RER) +  
fattyacid  I hope this helps to whomever was lost like me Null mutation: A mutation (a change) in a gene that leads to its not being transcribed into RNA and/or translated into a functional protein product. For example, a null mutation in a gene that usually encodes a specific enzyme leads to the production of a nonfunctional enzyme or no enzyme at all. +1