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 +17  (nbme22#5)

The concept is a convoluted way of asking if you knew how VDJ recombination works, which is that it is actually an example of altering the DNA of the B/T lymphocyte.

Southern blot technique: So when they use a probe against some region, and outputting a size of 1.5 kb or 6 kb, this is telling you the size of the DNA fragment in each cell (doesn’t matter if they say J probe or constant region probe, they’re just saying they’re targeting some nucleotide sequence found in the Ig locus/TCR beta chain locus respectively for B/T cells).

I think the confusing part could be wondering how you know whether you’re partly through rearrangement (answer choices B thru D) or if it hasn’t occurred at all yet (correct answer). Here, the concept is that B cells undergo V(D)J rearrangement in the bone marrow, while T cells do it in the thymus, and it all happens at once. So a plasma cell in the blood like in Multiple Myeloma would have fully undergone recombination, while a T cell in the blood could either be fully educated (and have finished VDJ recombination) or immature (hasn’t started VDJ).

Since the T cell gene was 6 kb and definitely bigger than the 1.5 kb gene, the T cell hasn’t undergone recombination yet.

trichotillomaniac  very nice explanation!
nwinkelmann  This was awesome! Made so much sense and hopefully I will be able to think that critically about questions in the future (because I NEVER would have come up with this on my own, hah).
eacv  OMG! THANK YOU. I DIDNT KNOW ANYTHING about this!! Hope this is not testesd on real examen :p




Subcomments ...

Why wasn’t the table enough to determine prevalence in the general population?

sacredazn  For the case control question, it’s taking that principle that you can’t use case control studies to calculate relative risk and applying it to prevalence. Basically with case control studies we start by saying okay, I’m going to find 200 people with sinusitis and 400 without. Then, you go back and look at the number exposed/unexposed and calculate the odds ratio. So you can’t use case controls to calculate prevalence because it all depends on how many cases you picked in the first place. Might make more sense to think about it with a rare cancer like craniopharyngioma or something- let’s say you chose 10 cases and 10 controls and wanted to look at how many people smoked. It wouldn’t make sense to then say the prevalance of craniopharyngioma is 10/20 = 50%. +6  
dr_trazobone69  Thank you, that makes a lot of sense! So we can use relative risk (cohort studies) to calculate prevalence? +  
sacredazn  @trazobone Hmm I think the wording would be key, you could use a prospective cohort to calculate incidence, but you wouldn’t be able to find prevalence of the gen population unless you had more info. I think the concept is that really to calculate prevalence you need a proper ecologic study looking at population-level data. The way it was worded in the question was tricky though lol since when has “cannot be determined from the info given” ever been a right answer. +3  
nwinkelmann  @sacredazn thank you! this was the best explanation to use the rare disease comparison. Made everything make so much sense and hopefully I'll actually just remember it now, instead of learning the factoid and failing to recall it all the time. +1  
hyperfukus  i guess this makes sense but i don't understand why we are asked to calculate it from tables like this then? is there more info in those? +  
hello  @hyperfukus The table was given because that a 22 table is typically what you do see regarding data for case-control studies. If the 22 table wasn't include, then literally everytone would pick Choice "E" as the correct answer b/c you can't calculate something without being provided numbers. The difference in including the data-table is that 1. again, you need to report a 22 table because that is typically what you will see regarding data for a case-control study and 2. by including the 22 table, it actually tests if the test-taker realized that the data in the 2*2 table does not help at all with calculating prevalence-- because case-control studies NEVER report on prevalence. +  


Why wasn’t the table enough to determine prevalence in the general population?

sacredazn  For the case control question, it’s taking that principle that you can’t use case control studies to calculate relative risk and applying it to prevalence. Basically with case control studies we start by saying okay, I’m going to find 200 people with sinusitis and 400 without. Then, you go back and look at the number exposed/unexposed and calculate the odds ratio. So you can’t use case controls to calculate prevalence because it all depends on how many cases you picked in the first place. Might make more sense to think about it with a rare cancer like craniopharyngioma or something- let’s say you chose 10 cases and 10 controls and wanted to look at how many people smoked. It wouldn’t make sense to then say the prevalance of craniopharyngioma is 10/20 = 50%. +6  
dr_trazobone69  Thank you, that makes a lot of sense! So we can use relative risk (cohort studies) to calculate prevalence? +  
sacredazn  @trazobone Hmm I think the wording would be key, you could use a prospective cohort to calculate incidence, but you wouldn’t be able to find prevalence of the gen population unless you had more info. I think the concept is that really to calculate prevalence you need a proper ecologic study looking at population-level data. The way it was worded in the question was tricky though lol since when has “cannot be determined from the info given” ever been a right answer. +3  
nwinkelmann  @sacredazn thank you! this was the best explanation to use the rare disease comparison. Made everything make so much sense and hopefully I'll actually just remember it now, instead of learning the factoid and failing to recall it all the time. +1  
hyperfukus  i guess this makes sense but i don't understand why we are asked to calculate it from tables like this then? is there more info in those? +  
hello  @hyperfukus The table was given because that a 22 table is typically what you do see regarding data for case-control studies. If the 22 table wasn't include, then literally everytone would pick Choice "E" as the correct answer b/c you can't calculate something without being provided numbers. The difference in including the data-table is that 1. again, you need to report a 22 table because that is typically what you will see regarding data for a case-control study and 2. by including the 22 table, it actually tests if the test-taker realized that the data in the 2*2 table does not help at all with calculating prevalence-- because case-control studies NEVER report on prevalence. +