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NBME 21 Answers

nbme21/Block 3/Question#15 (39.6 difficulty score)
A 47-year-old man has jaundice. Laboratory ...
Obstruction of the bile duct🔍
tags: bilirubin 

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submitted by hungrybox(800),
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wyh omyehssli si rn:gow

erhTe uohlds mtoasl vrene eb sthrtaig pu iilbrbuni in eht rne.ui nI smysiloeh, het excses ruilinbbi si eetdrxce ni hte i.elb fertA aitlearcb ecvionorsn nad tua,eekpr esmo lilw be etxredec ni eht ueirn as iiolbr.un ,eerovHw in trivuoscteb sdsder,iro het jodtgnuace birlbuini lilw nrvee aveh the inypoouprtt to gduroen itbrlacae nvnrcsoieo to icebtioun/rlsr.o nI tish aw,y eth nacdgoetju iiruibbnl sha no rehto way to eb eetedrxc htroe ahnt declytri ni eht i.renu

ertidsc to 6cl3n7//auaar on iddter

skip_lesions  Found a good pic showing bilirubin metabolism +  

submitted by haliburton(192),
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i knthi ihst si sceuabe inbluribi si a oleulbs revil krndeowab udcprto fo h,eme tbu ash ton etrened eht tenlo/etnsciino ofr gut ecbaatir ooeivncrsn to inobctleisr ro lnb.urioi iloinrub ni euinr si

submitted by peridot(40),

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 basic_pathology(5),

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) +