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NBME 20 Answers

nbme20/Block 4/Question#20

A 5-year-old boy is brought to the physician for a ...


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 +4  upvote downvote
submitted by celeste(21),

This sounds like Fanconi syndrome. The proximal tubular epithelial cells have a hard time reabsorbing filtrate, so you'll see a loss of phosphate, amino acids, bicarbonate, and glucose.

medschul  Wouldn't Fanconi syndrome also cause hypokalemia though? +2  
yotsubato  Especially considering the fact that the DCT will be working in overdrive to compensate for lost solutes??? +  
nala_ula  This question did not make sense to me at all. I knew it was Fanconi syndrome yet didn't select the obvious answer because it said "follow up examination 1 week after diagnosis". I thought it would already be in treatment... I searched (now) and it says that treatment is basically replenishing was is lost in the urine. So definitely the wording is like wtf to me +  
sugaplum  I was thinking since it affected the PCT that Na resorption would be affected as well? But I guess the other segments will pick up the slack? +  

 +0  upvote downvote
submitted by imgdoc(9),

You can cross out the top three answer choices, A,B,C. You wont be reabsorbing anything in the PCT in fanconi's syndrome. Looking at hypokalemia, hyponatremia, and hypophosphatemia now. Hypokalemia can't be correct because even though potassium is lost it will be reabsorbed at the later thick ascending loop and if that doesnt make sense, the body will adjust for low serum potassium but activating the H+/K+ pump on cells. It isn't hyponatremia because at the collecting duct principal cells, reabsorption will occur. This leaves hypophosphatemia as the correct and only answer choice.

imgdoc  by* +1  
larascon  Excellent explanation, thank you ! +  

 -2  upvote downvote
submitted by dontwanttofail(-2),

The answer is actually hypokalemia, according to NBME.

dontwanttofail  JK. Please ignore. The answer is hypophosphatemia +