NBME 20 Hypophosphatemia (NBME Answers)

Retired NBME 20 Answers

nbme20/Block 4/Question#20 (reveal difficulty score)
A 5-year-old boy is brought to the physician ...
Hypophosphatemia 🔍 / 📺 / 🌳 / 📖
tags: renal

+30 upvote downvote
submitted by ∗imgdoc(183)

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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.

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imgdoc by* +2

larascon Excellent explanation, thank you ! +2

waterloo It's worth mentioning, that 25% of Na is reabs in the thicc ascending limb. 67% of filtered Na is reabsorbed in the PCT, whereas 85% of phosphate reabsorbed in the PCT. So pt more likely to show Hypophos. +4

coconut UWorld QID:7626 says lab results in people with Fanconi syndrome will show hypokalemia +

j44n coconut i thought that too but then i remebred fanconi is associated with hypophosphatemic rickets/ VIT D resistant rickets and this little dude is going to be growing because he's 5 and thats gonna increase the demand for phosphate in his body +1

habs741 Many sources (Medscape, UpToDate, random journal articles) all describe Fanconi presenting with hypokalemia. I believe the mechanisms is loss of PCT reabsorption of Na causeing increased Na delivery distally where it needs to be reabsorbed. In the collecting duct that will result in K wasting. So hypokalemia and hypophosphatemia are both true, which makes this a bad question IMO. But I could see NBME arguing that hypophosphatemia is the "best" answer because it's more directly related to the PCT defect and always true (vs hypokalemia which is not necessarily present and is indirectly related to the PCT defect.) Still think it's a lame question though. +

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submitted by ∗celeste(96)

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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.

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medschul Wouldn't Fanconi syndrome also cause hypokalemia though? +4

yotsubato Especially considering the fact that the DCT will be working in overdrive to compensate for lost solutes??? +1

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 +1

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? +

-7 upvote downvote
submitted by ∗mdmikek89(6)

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Y'all over complicate this shit so hard. Whether or not its Fanconi, it says Proximal Tubule defect. 85% of phosphate is reabsorbed here. If its defective, phosphate re-absorption is decreased.

hypophosphatemia

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meddoc2023 Yes and sodium absorption would be affected too I believe because the PCT is affected +

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