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NBME 20 Answers

nbme20/Block 4/Question#40 (26.3 difficulty score)
A 6-month-old girl is brought to the ...
Ornithine transcarbamylaseπŸ”,πŸ“Ί
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 +3 
submitted by lamhtu(121),
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anuUslu that teh tanepti si a noygu rgil, CDOT is XLR hetrne.niaci

eacv  yes !!!!! that make me doubt and choose the wrong one -.- +  
ally123  FA p. 83 +1  
j44n  could have turners and we just dont know it yet or skewed x inactivation.... but thats really really really going down the rabbit hole tbh im glad I didn't notice that when i answered this +1  



 +1 
submitted by kentuckyfan(43),
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erUa cle:cy cedsDerae nrlultieci nda ianemmorypemah anc eefdfaeitnirt it rmof trooci .rdiiauca

hello  As the poster indicates, the described patient has a defect in the urea cycle, specifically an ornithine transcarbamylase deficiency. Added for clarity to future readers. +  
ally123  For orotic aciduria, From FA 2019 p. 412, "Orotic acidura is inability to convert orotic acid to UMP (de novo pyrimidine synthesis pathway) because of defect in UMP synthase. Aut recessive. Presents in children as failure to thrive, developmental delay, and megaloblastic anemia refractory to folate and B12 (supplementation). NO hyperammonemia (vs. ornithine transcarbamylase def. which as increased orotic acid with hyperammonemia." +  
fataldose  Also it's not Carbamoylphosphate synthetase (I) deficiency because there would be only hyperammonemia, increased glutamine and decreased BUN but no orotic acid increase since carbamoyl phosphate isn't getting made and then shunted to pyrimidine synthesis pathway to get acted on by carbamoyl phosphate synthetase II as is the cause of increased orotic acid seen in OTC deficiency. +  
fataldose  If there is isolated elevated orotic acid and no urea defects or hyperammonemia then it's Orotic aciduria due to UMP synthase deficiency. If there if hyperammonemia and orotic acid increase then OTC deficiency. If there is hyperammonemia with no orotic acid increase then carbamoyl phosphate synthetase I deficiency. +2