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NBME 24 Answers

nbme24/Block 3/Question#24

A 3-month-old boy is brought to the emergency ...

Gluconeogenesis

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The disease here is fructose bisphosphatase deficiency. In it, IV glycerol or fructose doesn’t help because both enter the gluconeogenesis pathway below fructose bisphophatase. Galactose on the other hand enters above it. I don’t think you really need to know this to choose the correct answer since the clinical picture of fasting hypoglycemia that is corrected w/ some sort of sugar that can enter the gluconeogenesis pathway should clue you into the right answer.

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. +9  
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! +10  
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 +1  
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. +  




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submitted by usmle11a(13),

guys VG would worsen with galactose infusion, remebmber they dont have G6Pase which means they cant convert anything to glucose