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NBME 22 Answers

nbme22/Block 4/Question#14 (45.1 difficulty score)
In patients with adenosine deaminase ...
Ribonucleotide reductaseπŸ”

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submitted by haliburton(208),
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stih egiam shosw iohcplyytmc c.xottiyi

tsih megia oshsw ahtt hte bliuupd fo ATdP ihsiibnt RRN tlooudrn(eebcii es,u)tacrde ihchw siinbthi teh srte dNPT ssiy.hsent

nusbo ieagm shsow RNR is the trtgae of .ourryhxdeya

leaf_house  That second image is solid gold. +3  

submitted by beanie368(19),

I did not know the mechanism of how dATP is harmful, although I knew that it was lymphotoxic. You can check out U world question ID: 15293 for a pretty good explanation

submitted by pseudo_mona(14),

If you just remembered the fact that RNR converts ribonucleotides to deoxyribonucleotides, it makes sense that excess dATP (a deoxyribonucleotide) would inhibit the enzyme

submitted by coolcatac(18),
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submitted by neovanilla(35),

Got this wrong, but when looking this up, I saw that SCID is associated with a lack of adenosine deaminase --> accumulation of adenosine, which is lympho-toxic.

submitted by waterloo(71),

ADA def causes increase of dATP which strongly inhibits ribonucleotide reductase, reducing dNTP needed for DNA synthesis.

This would particularly effect lymphoid proliferation and function that relies heavily on nucleotide biosynthesis. Half of the cases of neonatal onset of SCID are due to ADA def.

submitted by usmleboy(9),

So I missed this question and I was very confused at first as to why. When I first read this question, I thought of Cladribine. Cladribine (Hairy Cell Leukemia ttx) is an adenosine deaminase inhibitor. Cladribine inhibits DNA polymerase, and I chose DNA Polymerase I as my option. Why was I wrong?

Because DNA polymerase I is the prokaryotic DNA poly, not in human lymphocytes. I glanced over the "I" when I was taking this test. If you see this question on your exam, and DNA polymerase is an option - THATS the answer, but ribonucleotide reductase would be a more indirect inhibitor of DNA.

The question also asks for DNA synthesis specifically, which further suggests RR.

These NBMEs do be tricky.