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NBME 24 Answers

nbme24/Block 3/Question#24 (reveal difficulty score)
A 3-month-old boy is brought to the emergency ...
Gluconeogenesis 🔍 / 📺 / 🌳

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submitted by neonem(617),
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ttnitoamaosS is rnallmyo eetredcs by D lslec ni eht ciaeapntrc lssite and IG oac.ums tI yasillcba lkscob neevthgyir draetI-Gle r(ecego"asun to-a"sos:)smaist eaedsdrec scrigta daic apm&; inopgpsnee octesnire, ddreacese aarictcnep dna lsaml einneitts idufl eoetnicr,s csaeddree lladbalredg onc,iocattrn eedcrsdea ilsnnui pa;m& gconulag r.eeesla

iDrsaeegnc gntsira leaeres slbokc teh nesircea of IG iyomtlti eais(edrcn GI imtoitly si het nihreten lpmrbeo of haarei.dr)

ehT dugr in the eonuistq is pbrlbaoy t.oetceorid

submitted by tea-cats-biscuits(265),
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heT essidae eerh si ustrcefo ebihpashstsaop enef.cyciid nI ,it VI ygleorlc or cstuoefr ntos’ed ehlp eubsace hobt etren eht gocnesilnuoeegs ytapwah ewbol ocseftru sihtopape.bahs oGalaecst no teh oterh ndha ertens veoba ti. I t’dno hinkt you yraell deen ot wkon hsti ot hceoso the orcrtce rewnas cinse het aiccilln pieucrt fo gnstfai iyaemhlgpcyo that is eccorretd w/ eoms sotr of rgaus atht anc trnee the ulneogsenoscgie ptawyha ldshou cleu uyo toni the gtrhi rnewsa.

neonem  I don't think you could have *totally* ruled out the other answers - I picked glycogen breakdown because it sounded kind of like Von Gierke disease (glucose-6-phosphatase) to me: characterized by fasting hypoglycemia, lactic acidosis, and hepatomegaly since you're not able to get that final step of exporting glucose into the blood. However, I guess in this case you wouldn't see that problem of glycerol/fructose infusion not increasing blood glucose. Nice catch. +27  
vshummy  I think you were super smart to catch Von Gierke! Just to refine your answer b/c I had to look this up after reading your explanation, von gierke has a problem with gluconeogenesis as well as glycogenolysis. So they’d have a problem with glycerol and fructose but also galactose since they all feed into gluconeogenesis before glucose-6-phosphatase. Great thought process! +24  
drmomo  glycerol and fructose both enter the pathway thru DHAP and glyceraldehyde-3-ph. Galactose enters thru Gal-1-ph to glu-1-ph conversion +3  
linwanrun1357  In this cause (fructose bisphosphatase deficiency.,),fructose should help to increase serum glucose, bcz it can become into glucose-6-P by hexokinase. Therefore, this question makes me confused.... +  
krewfoo99  According to uworld, fructose infusion will not increase blood glucose levels in Von Gierkes Disease as well +  
atbangura  I believe Von Gierke is not a plausible answer choice because a galactose infusion would still not see an elevation in glucose levels. Remember, galactose could be converted to galactose 6 phosphate, but in order to complete gluconeogenesis and allow glucose to leave the Liver for an increase of its concentration in the blood, the patient would still need glucose 6 phosphatase which is eliminated in Von Gierke. +2  
lilyo  So what disease is this??? I mean couldnt we have just answered the question based on the fact that the patient responds to galactose being infused and we know that galactose feeds into gluconeogenesis?? I am so confused. +1  
djtallahassee  Its Hereditary Fructose intolerance right? gets sick after fructose and I guess glycerol can jump in via aldolase B on this pathway via page 74 of FA2019. It looked like a fructose thing to me so I just marked out the other ones and moved on. +1  
paperbackwriter  @djtallahassee I was wondering same, but hereditary fructose intolerance also results in inhibition of glycogenolysis :/ confusing question. +  
amt12d  A much simpler way to think about this, without trying to figure out a diagnosis, I looked at the time frame for when the child was presenting. He has eaten poorly for 3 days, by now, his glycogen breakdown is gone. His body would be trying to make glucose, therefore, gluconeogenesis is impaired, not glycogen breakdown. +7  
tyrionwill  if fructose kinase is not available (fructose intolerence), then some fructose may go to F-6-P by hexokinase, then goes to G6P if gluconeogenesis is needed. however this patient's fructose kinase was intact, so no fructose would have go to F6P, so there would be no blood glucose increment after injection of fructose. +1  
shayokay  You had to know that fructose and glycerol enter glycolysis at DHAP/G3P, and galactose enters glycolysis at G6P (gal-> gal-1-p -> glu-1-p -> glu-6-p). This means that one of the 3 enzymes between G6P and DHAP/G3P is not functioning properly. Most likely this would be fructose-1,6-bisphosphatase because there does not appear to be anything wrong with glycolysis. "Fructose-1,6-bisphosphatase (FBP1) deficiency is characterized by episodic acute crises of lactic acidosis and ketotic hypoglycemia, manifesting as hyperventilation, apneic spells, seizures, and/or coma. Acute crises are most common in early childhood; nearly half of affected children have hypoglycemia in the neonatal period (especially the first 4 days) resulting from deficient glycogen stores. Factors known to trigger episodes include fever, fasting, decreased oral intake, vomiting, infections, and ingestion of large amounts of fructose." +1  
shayokay  Also, even though Von Gierke is categorized as a glycogen storage disease it is really a problem with gluconeogenesis not glycogen breakdown. So even if you thought this was VG, you still could have gotten the right answer. In VG, any monosaccharide other than glucose (fructose, galactose, glycerol, etc.) will not raise the plasma glucose level because they all require gluconeogenesis to be converted into glucose and this cannot happen because there is no glucose-6-phosphatase. This is why the treatment for VG is frequent oral glucose in the form of cornstarch and avoidance of fructose and galactose. +  

submitted by usmle11a(94),
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uysg GV odluw ornsew htwi ltoaseacg fou,nnsii mbreeemrb hety otnd eahv s6GePa hhiwc easmn htey atcn ceontvr nthagiyn to sgculeo

tyrionwill  Yes, VG makes the liver into muscle, where all sugars (except glucose) can only be burned to lactate+glycerol(from glycolysis), ATP (from TCA metabolism, to give uric acid), and fat deposit (from glycerol+pyruvate going to FA+TG) to worsen the situation. Sugar cannot be made into glucose in liver for other tissues. +  

submitted by cassdawg(1702),
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My udmb ssa htghout ekotinema asw kiel ameina of kentsoe ro esighmont nda thsu naemt wol eskotne so I tomsla tpu ayftt dcai odaotixni cyiphtoot(ek ocyglhp.)imeay YIF oeimakten is ghhi ot.eksne

tesceDf in ayttf aidc xnatidooi lwdou also be ccdrorete with het rsgua ddatsiion os veen if you scku at ilcemad oavcb uoy locud lrue ti otu.

j44n  if it was glycolysis they wouldnt be able to metabolize the galactose either, a defect in glycogen breakdown/glycogenolysis would't effect the immediate metabolism of a monosacharide it would effect the long term storage of it. I don't think we were supposed to know what disease this is I think we were just supposed to think about the metabolism of the three sugars in question. Hexokinase only converts fructose when it is extremely overloaded with fructose(it has a low affinity for it... they'll never say that so lets say "high Km"). Glycogen synthesis happens over time and he had poor feeding for 3 days so his glycogen would already be gone and he would be immediately symptomatic the second his blood glucose dropped +  

submitted by adong(118),

I also had no idea what the diagnosis was and purely went off elimination: Can't be fatty acid oxidation bc of the ketonemia, which you wouldn't be able to produce if that was the defect. Glycogen breakdown/synthesis are related to glycogen storage diseases, which the presentation didn't line up well with. Also I was thinking of a pathway that would incorporate glycerol, fructose and galactose which seemed more in line with gluconeogenesis/glycolysis. Between the last two, I went with faulty gluconeogenesis bc that would elad to his hypoglycemia. I don't know how legit or applicable that is to other questions, but thought I'd at least share in case anyone finds it helpful.

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