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NBME 21 Answers

nbme21/Block 3/Question#15 (43.5 difficulty score)
A 47-year-old man has jaundice. Laboratory ...
Obstruction of the bile ductπŸ”
tags: bilirubin 

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submitted by hungrybox(977),
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yhw miylesosh is rnwg:o

heeTr huodsl matols nerve be ghttsria pu nibuiribl in teh .nrieu In ,slyhseoim hte ssexce niirbubli si receetdx in het Aeftr ilebcrtaa voorcnisne adn tpekuer,a smeo will be eextcder ni the uneri sa n.lbriiou eer,oHvw in tcuistebvor rdso,iedrs hte tonajcdegu rulinbbii llwi erven ahev eht prtopotnuiy ot degnour lraebicta ennvoiscro ot ocn/ero.sirtubil nI stih y,wa eth aujneogctd ilirnubib ahs on hteor wya to eb cexeedtr hreot nath tlycdeir in teh ne.riu

iecdtrs to 3a7/aan6ru/lc on tdired

skip_lesions  Found a good pic showing bilirubin metabolism +  

submitted by peridot(59),

Here my stab at summarizing all the info. The first diagram on this website is a super helpful as well; This diagram sorta helps too.

Normally, bilirubin from the blood stream is conjugated in the liver to a water-soluble form. This gets mixed with bile to be excreted through the intestines. In the small intestine, it becomes urobilinogen, some of which is excreted by the kidneys, giving pee that good yellow color. Some of the urobilinogen turns into stercobilin and stays in the GI to be excreted in poop, giving poop that good brown color.

In bile duct obstruction, bilirubin gets conjugated to a water-soluble form - then it's stuck. It's unable to be excreted into the intestines, unable to turn into urobilinogen or stercobilin, etc. Conjugated bilirubin builds up in the liver, and eventually gets backed up back into the blood stream where it came from. Since it's water-soluble and floating around in circulation, it eventually gets filtered by the kidneys and ends up in pee. Bilirubin is darker in color, so the pee would be dark. However, the stool would be pale since no stercobilin was made. I believe that symptoms of itchy skin also occurs in these cases as bile salts back up and are deposited in skin.

Hemolysis is not the correct answer because most of the bilirubin is not yet conjugated, so it's not water-soluble and doesn't end up in the kidneys.

submitted by haliburton(209),
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i nikth isht si ubaesce ibrliunib si a sbuolel vreli kdorwenba ortcupd fo ,emhe ubt has ton eendert eth etioenco/innslt ofr utg catarbei isrcvonone to bicoeirnslt ro nilobr.ui rnuobili ni irune si nramo.l

submitted by basic_pathology(8),

Key Point: Bilirubin can only be in the urine if it is: (1) conjugated BR, or (2) urobilinogen.

Unconjugated BR is NOT water soluble and therefore CANNOT be in urine. This is why you use phototherapy in Crigler-Najjar. Increased unconjugated BR --> phototherapy isomerizes it so it becomes water soluble.

lovebug  clear and short. good! THX +  
lovebug  as @Basic_pathology said, In Crigler-Najjar synd. Photo TX does not conjugate UCB, but does increase Polarity and Water solubility to allow excreation (FA 2019, 388PG) +