Which phase in hepatic metabolism is responsible for conjugating drugs for excretion?

• A. Phase I oxidizes drugs
• B. Phase II deactivates drugs
• C. Phase I stores drugs
• D. Phase II conjugates drugs for excretion

Answer: (D)

Conjugation for excretion is the job of Phase II in hepatic metabolism. In this stage, the liver attaches polar groups such as glucuronide, sulfate, acetate, or glutathione to the drug or its Phase I metabolite. These additions greatly increase water solubility, making the compound easier to eliminate in urine or bile. Phase I, by contrast, modifies the molecule to reveal or create functional groups through oxidation, reduction, or hydrolysis but does not accomplish the conjugation needed for excretion. Descriptions that Phase II merely “deactivates” drugs or that Phase II is for storage don’t accurately capture the primary purpose, which is to form excretable, more water-soluble conjugates.