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NBME 24 Answers

nbme24/Block 1/Question#50 (55.5 difficulty score)
An 11-year-old boy is brought to the ...
Intracellular and extracellular dehydrationπŸ”

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submitted by vshummy(156),
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fulminant_life  I just dont understand how that is the cause of his altered state of consciousness. Why wouldnt altered affinity of oxygen from HbA1c be correct? A1C has a higher affinity for oxygen so wouldnt that be a better reason for him being unconscious? +6  
toupvote  HbA1c is more of a chronic process. It is a snapshot of three months. Also, people can have elevated A1c without much impact on their mental status. Other organs are affected sooner and to a greater degree than the brain. DKA is an acute issue. +5  
snafull  Can somebody please explain why 'Inability of neurons to perform glycolysis' is wrong? +3  
johnson  Probably because they're sustained on ketones. +3  
doodimoodi  @snafull glucose is very high in the blood, why would neurons not be able to use it? +1  
soph  @snafull maybe u are confusing bc DK tissues are unable to use the high glucose as it is unable to enter cells but I dont think thats the case in the neurons? +1  
drmomo states its primarily due to acidosis along wth hyperosmolarity. so most relevant answer here would be dehydration +1  
drmohandes  I thought the high amount of glucose in the blood (osmotic pressure), sucks out the water from the cells. But you also pee out all that glucose and water goes with it. That's why you have to drink and pee a lot.. +7  
titanesxvi  Neurons are not dependent on insulin, so they are not affected by utilization of glucose (only GLUT4 receptors in the muscle and adipose tissue are insulin dependent) +24  
drpatinoire  @titanesxvi You really enlightened me! +  
mutteringly  I don't make the connection of what titanesxvi said to the question - can someone explain? +  
motherhen  @mutteringly it explains why the answer choice "inability of neurons to perform glycolysis" is wrong +1  

submitted by usmle11a(73),
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syas atht rthoysyamliepor nda asdcisio rltae nlatme au.sstt

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p.s i ogt ti wrgno too :)

submitted by hungrybox(976),

Big Robbins:

Idk how you could say that it's from extracellular dehydration, but whatever I guess.

submitted by j44n(42),

this is how I looked at it extra cellular osmoles> intracellular so it will pull the h20 out.... then the high osmotic pull of the sugar overwhelming the SGL2 transporter in the kidney will pull the h20 out of the body dehydrating the extracellular compartment

j44n  also the brain/CNS/neurons have the highest affinity GLUT transporters.... thats why giving glucagon brings someone out of hypoglycemia when theyre passed out. The glucagon increases the blood sugar and it goes straight to the brain restoring concious +  

submitted by waterloo(71),

a little messed up, but "Inability of neurons to perform glycolysis" seems like a tempting answer. But the reality is, the neurons are able to perform glycolysis, they ready to rock but just waiting on insulin. I still chose this as my answer tho.

I guess this is one of those choose the best answer questions. I think FA should add the reasoning behind cerebral edema, being that it's a major cause of death (but I couldn't find it in Robbins either). Having so much glucose in the blood vessels causes water to be drawn out (ICF --> ECF). So that's a intracellular dehydration.