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NBME 24 Answers

nbme24/Block 2/Question#1

A 73-year-old woman has had easy fatigability for 2 ...

Failure of conversion of N5-methyltetrahydrofolate to tetrahydrofolate

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submitted by lsmarshall(199),

Although Crohn's may have ulcers, fistulas, and bleeding; it usually does not cause iron deficiency anemia and has less bleeding than UC. "Structural abnormalities of the terminal ileum, such as Crohn disease and surgical resection, can cause decreased absorption of vitamin B12." - First Aid General Principles





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submitted by yex(16),

According to Goljan: "B12 (cobalamin) has cobalt in it. Circulating form of folate is methyltetrahydrofolate. Purpose of cobalamin (B12) is to take the methyl group off of methyltetrahydrofolate. Then it’s called tetrahydrofolate. If you don’t get the methyl group off of folate, you will not make DNA. So, if you are B12 def, you can’t get the methyl group off and cannot make DNA. If you are def in folate, you can’t make DNA."





Why would it not be anemia of chronic disease with decreased serum transferrin concentration?

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!) +7  
step1forthewin  Hi, can someone explain the blood smear? isn't it supposed to show hypersegmented neutrophils if it was B12 deficiency? +  
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 +8  
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... +  
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.... +  
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. +2  
sahusema  I didn't even register that was a lymphocyte. I thought I was seeing target cells so I was confused AF +  




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





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.