Final answer:
Conjugated (direct) bilirubin is water soluble because it is converted to bilirubin diglucuronide in the liver, making it excretable in bile.
Step-by-step explanation:
The question pertains to whether conjugated (direct) bilirubin is water soluble or not. Conjugated bilirubin is indeed water soluble. This is because in the liver, bilirubin is conjugated with glucuronic acid to form bilirubin diglucuronide by the enzyme UDP glucuronyl transferase. The conjugation makes bilirubin water soluble, allowing it to be easily excreted. The Direct Van Den Bergh test indicates this solubility by producing a purple-red color when conjugated bilirubin reacts with Diazo reagent, confirming that it is water soluble and can be excreted without the need for alcohol to enhance the reaction.