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NBME 24 Answers

nbme24/Block 4/Question#44 (37.8 difficulty score)
A 71-year-old woman with non-Hodgkin lymphoma ...
tags: oncology 

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submitted by sajaqua1(558),
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submitted by pfebo(5),

Easier way to approach question. Look at the creatinine value it is elevated, Methotrexate is renally excreted so we can preview an increase in levels due to impaired renal excretion which leads to chemotoxicity. Leucovorin is a common chemoprotectant used to avoid Metotrexate toxicities.

submitted by medstruggle(12),
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yWh od ouy egvi VI niecrvuool iwth ltheanrctai et?ahotxermte otu’dnlW MTX sole tis iyeafcfc sneci veoniolucr rserevse hte tcefsef of ?TMX

colonelred_  MTX will still work but yes some purine/pyrimidine synthesis can still occur. You often give leucovorin to decrease adverse effects of MTX. +  
welpdedelp  ok I have a question, leucovorin is the same as folic why give one over the other? +1  
lsmarshall  Leucovorin, folinic acid, is a modified version of THF and enter folate metabolism where THF is, after the point where methotrexate takes its effect. I have a pharm. card that says "toxic effects on normal cells may be reduced by administration of folinic acid (a.k.a. leucovorin or citrovorum factor), which is **preferentially taken up by normal cells versus tumor cells**." +14  
jj375  @welpdedelp I also struggled with that and chose the wrong one, but then I remembered something from biochem that I think is helpful:I think the good thing about leucovorin is that it doesn't need to be activated by dihydrofolate reductase. Methotrexate is directly blocking Dihydrofolate reductase, so folate can't be activated to help, but leucovorin doesnt need the activation, it is turned into Tetrahydrofolate and rescues the cell! +