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NBME 24 Answers

nbme24/Block 2/Question#1 (41.7 difficulty score)
A 73-year-old woman has had easy fatigability ...
Failure of conversion of N5-methyltetrahydrofolate to tetrahydrofolate🔍

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 +8  upvote downvote
submitted by yex(61),
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cgcoAnird to ojlG:an 1"2B a()labmionc hsa bcltoa ni ti. trCcnuiilga mofr of aoeflt is dyret.rhhtayfoetamoltle peusroP fo cmaabnoil 1)(2B si ot taek hte hteyml group fof of oyrtrhtfy.leamlhatoeedt nehT s’it alclde .haelotfyretaotrd fI yuo o’ntd get het ythlme ropgu off of etloaf, ouy illw tno meak .AND ,So fi uyo rea 12B f,de ouy c’tna gte teh lethmy rogup ffo dan taconn akem .NAD fI oyu are efd in alefto, uyo an’ct mkea .DN"A

misterdoctor69  lmao +  

pic of the methionine/folate cycle:

+2/- hungrybox(771),

 +7  upvote downvote
submitted by lsmarshall(345),
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hlhgotAu rsCn'oh aym veha e,rulcs ifa,tslsu dan legn;dibe it lulsyau dseo tno useac niro fcieidecyn mnieaa dna has elss ilegnbed tnah U.C rauSu"tcltr tsnmlorbeiaia of the miterlan emui,l hcsu sa nCohr edisase dan lcruiags s,noeiecrt can caesu eeasdedrc abrotipnos of matnivi 2".B1 - isFrt Adi reGlnea iPrpceslni

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yhW ulowd it ont be aeianm fo hrccoin aesedsi htwi caeddeers rsume afnreitrnsr ttoanncicorne?

lispectedwumbologist  Nevermind I'm stupid as fuck I see my mistake +  
drdoom  be kind to yourself, doc! (it's a long road we're on!) +17  
step1forthewin  Hi, can someone explain the blood smear? isn't it supposed to show hypersegmented neutrophils if it was B12 deficiency? +1  
loftybirdman  I think the blood smear is showing a lone lymphocyte, which should be the same size as a normal RBC. You can see the RBCs in this smear are bigger than that ->macrocytic ->B12 deficiency +18  
seagull  maybe i'm new to the game. but isn't the answer folate deficiency and not B12? Also, i though it was anemia of chronic disease as well. +  
vshummy  Lispectedwumbologist, please explain your mistake? Lol because that seems like a respectible answer to me... +5  
gonyyong  It's a B12 deficiency Ileum is where B12 is reabsorbed, folate is jejunum The blood smear is showing enlarged RBCs Methionine synthase does this conversion, using cofactor B12 +  
uslme123  Anemia of chronic disease is a microcytic anemia -- I believe this is why they put a lymphocyte on the side -- so we could see that it was a macrocytic anemia. +  
yotsubato  Thanks NBME, that really helped me.... +1  
keshvi  the question was relatively easy, but the picture was so misguiding i felt! i thought it looked like microcytic RBCs. I guess the key is, that they clearly mentioned distal ileum. and that is THE site for B12 absorption. +6  
sahusema  I didn't even register that was a lymphocyte. I thought I was seeing target cells so I was confused AF +  
drschmoctor  Leave it to NBME to find the palest macrocytes on the planet. +4  

 +1  upvote downvote
submitted by krewfoo99(76),

folic acid when it enters the body is in the form of methyltetrahydrofolate (THF methylated). It donates its methyl group to vitamin B12 to become THF (the active form which as a DNA precursor). The vitamin B12 with its methyl group goes on to combine with homocyteine to form methionine.

macrocytic anemia (seen in blood smear) = RBC precursors can grow but not divide

oval macrocytes (seen in blood smear) = megaloblastic anemia (if they don't show neutrophil hypersegmentation)

B12 has neurologic symptoms and low methylmalonic acid while folate deficiency does not

folate is absorbed in jejunum, B12 in ileum

causes of B12 deficiency = pernicious anemia, gastrectomy, ileal resection, fish tapeworm, veganism

D) failure of MTHFR --> THF (via methyltransferase = B12 deficiency and macrocytic anemia

B) G6PD deficiency = normocytic because of incr RBC destruction; pt would show intravascular hemolysis sx precipitated by an oxidative stressor

C) protoporphyinogen oxidase deficiency = microcytic anemia because of low heme synthesis

A) decr transferrin = high ferritin = anemia of chronic disease = microcytic anemia because of low iron