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Retired NBME 23 Answers

nbme23/Block 2/Question#35 (reveal difficulty score)
A 42-year-old man with a history of recurrent ...
Magnesium ammonium phosphate ๐Ÿ” / ๐Ÿ“บ / ๐ŸŒณ / ๐Ÿ“–
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submitted by โˆ—krewfoo99(115)
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Image shows Stahorn Calculus

1) Staghorn Calculus in adults - Manesium Ammonium Phosphate 2) Staghorn Calculus in Children - Cystine

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cuteaf  The history of recurrent pyelonephritis + fever is also a giveaway that this was due to a bacterial infection. +1
an1  I thought the image looked like stag horn, but the pH is acidic, and MAP is alkaline! whyyy do they do this, literally mixing up incorrect info (unless im wrong, in that case lmk) +
bcher  normal urine pH is 4.5-8. Seems to me that 7.3 is on the higher side. +2



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submitted by chetzy(0)
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increase ph shows its magnesium ammonium phosphate crystals.ph 7.3

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bulgaine  The image shows a staghorn calculi which content is usually magnesium ammonium phosphate (can also be cystine but pH would be decreased not increased) and is caused by urease positive bugs that hydrolyze urea to ammonia leading to urine alkalinization (page 586 of FA 2019) +1
nukie404  Patient also has a fever, suggestive of a UTI +1
sigecapps  Why not calcium phosphate, also at high pH? I was between the two but decided on calcium phosphate because the images of staghorn I've seen seem to be more smooth and rounded. I guess should've gone more based on the pyelonephritis symptoms as well caused by the stone harboring bacteria. +1
jaramaiha  The high pH is the best thing to assess in urine which would indicate that it's an infection w/ proteus mirabilis, which is urease(+). FA20 pg 528 +1



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