why hemolysis is wrong:

There should almost never be straight up bilirubin in the urine. In hemolysis, the excess bilirubin is excreted in the bile. After bacterial conversion and reuptake, some will be excreted in the urine as urobilin. However, in obstructive disorders, the conjugated bilirubin will never have the opportunity to undergo bacterial conversion to sterco/urobilin. In this way, the conjugated bilirubin has no other way to be excreted other than directly in the urine.

credits to /u/alacran763 on reddit

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.

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.

i think this is because bilirubin is a soluble liver breakdown product of heme, but has not entered the intestine/colon for gut bacteria conversion to stercobilin or urobilin. urobilin in urine is normal.