NBME 23 Decreased sodium bicarbonate reabsorption in the proximal tubule (NBME Answers)

Retired NBME 23 Answers

nbme23/Block 4/Question#28 (reveal difficulty score)
An 18-month-old girl is brought to the ...
Decreased sodium bicarbonate reabsorption in the proximal tubule 🔍 / 📺 / 🌳 / 📖
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+28 upvote downvote
submitted by ∗seagull(1933)

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This is a type II Renal Tubular Acidosis. My Medical School Never taught this to me. Did you also go to poverty med school? I'm surprised they even gave us toilet paper.

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mousie haha mine didn't either. But they usually leave out most high yield info so, to be expected I guess. +12

yotsubato I didnt have physiology in my medical school. None, zip, zero, none. Nor did I have biochem. They said "you learned all this shit in undergrad, youll memorize it again for step 1 and forget it promptly" and then just moved on. +9

jcmed In the Caribbean thats 1 thing we were given... lots and lots of toilet paper +5

+20 upvote downvote
submitted by ∗smc213(169)

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To be completely clear!

This patient has Cystinosis a rare autosomal recessive lysosomal storage disorder and most common cause of Fanconi syndrome in children. Cystinosis is systemic and leads to cystine crystal deposits in cells and tissues throughout the body.

Although Wilsons disease can lead to FS, the crystals in the corneas does not correlate with Wilsons disease.
More info: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4841061/

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highyieldboardswards Thank you! You are a legend for figuring this out! +

paulkarr Appreciate you. +

drzed And even if it was Wilson disease, it would have the exact same consequence leading to Fanconi syndrome. +3

abhishek021196 Fanconi syndrome Generalized reabsorption defect in PCT = Increased excretion of amino acids, glucose, HCO 3 – , and PO 4 3– , and all substances reabsorbed by the PCT May lead to metabolic acidosis (proximal RTA), hypophosphatemia, osteopenia Hereditary defects (eg, Wilson disease, tyrosinemia, glycogen storage disease), ischemia, multiple myeloma, nephrotoxins/drugs (eg, ifosfamide, cisplatin), lead poisoning. Polyuria, renal tubular acidosis type II, growth failure, electrolyte imbalances, hypophosphatemic rickets = Fanconi syndrome (multiple combined dysfunction of the proximal convoluted tubule). +2

+7 upvote downvote
submitted by ∗lamhtu(139)

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To be even clearer, this sounds like Fanconi syndrome, which has lead to Type II RTA

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+5 upvote downvote
submitted by ∗sympathetikey(1600)

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Keys were the:

-Glucosuria

-Phosphaturia

-Amino aciduria

Those should be re-absorbed by the PCT, so if they're not, Type 2 RTA.

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lamhtu To be even clearer, this sounds like **Fanconi syndrome, which has lead to Type II RTA** +12

yb_26 To be even clearer: Wilson disease => Fanconi syndrome => type II (proximal) RTA +

charcot_bouchard To be even clearer, you all have been pretty clear +

yng I don't thin this is Wilson (copper in descemet layer of cornea). This is cystinosis (crystal in the cornea) --> Fanconi Syndrome --> Type II (PCT) RTA. +

+3 upvote downvote
submitted by ∗nor16(70)

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just know where the big stuff / important stuff is resorbed, it is the PCT. No need to know RTAs here...

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+1 upvote downvote
submitted by ∗asharm10(37)

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Phosphate is down that means something is wrong with PCT, then non anion gap acidosis that means bicarbonate is not being absorbed, glucosuria again PCT so just use your brain and choose decrease bicarbonate reabsorption in PCT

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+0 upvote downvote
submitted by ∗agraham416(5)

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I don't think you even had to know what disease this was, because I didn't. I noticed that the kid was losing a lot of things in the urine, so there's probably something wrong with the kidney. One of the main functions of the kidney is to either reabsorb bicarb and since she has a problem reabsorbing a lot of other stuff, I just figured lack of bicarb reabsorption made the most sense.

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+0 upvote downvote
submitted by ∗titanesxvi(106)

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why not decrease CA activity in the proximal tubule? this also could lead to metabolic acidosis.

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ergogenic22 carbonic anhydrase inhibitors can cause Type 2 RTA but it is not the cause here (cystinosis) +

doublethinker Yeah, I said CA too. Problem is that CA deficiency wouldn't lead to lack of reabsorption of all the ions listed. +1

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submitted by ∗charcot_bouchard(574)

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A case of Fanconi syndrome. If it was isolated Type 2 RTA option B would be the answer.

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-1 upvote downvote
submitted by ∗yb_26(315)

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Wilson disease => Fanconi syndrome => metabolic acidosis (type II (proximal) RTA)

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