Conjugated bilirubin should be in the highest concentration in the serum under normal conditions because it is water-soluble, allowing for excretion into the bile and feces, whereas unconjugated bilirubin is not water-soluble.
Step-by-step explanation:
In serum, the molecule that should be in the highest concentration is conjugated bilirubin. This is because unconjugated bilirubin, while it is water-insoluble, is transported to the liver bound to albumin to become soluble. The liver then conjugates the bilirubin with glucuronic acid to form bilirubin mono and di-glucuronide, making it water-soluble and allowing it to be excreted out of the body. Conjugated bilirubin is readily excreted into the bile and ultimately into the feces, while unconjugated bilirubin cannot be excreted as easily due to its lack of solubility in water. Hence, in a typical scenario where the liver and bilirubin excretion processes are functioning normally, conjugated bilirubin levels would be higher in the serum as compared to unconjugated bilirubin.
However, in certain medical conditions, such as hepatic jaundice or obstructive jaundice, the levels of both conjugated and unconjugated bilirubin can be elevated in the blood, but these are pathological situations. Normally, conditions like obstructive jaundice will lead to an increase in conjugated bilirubin in the blood due to an obstruction in bile flow.