Pharmaceutics Drug Disposition Practice Test

Xenobiotics are substances foreign to the body. An endogenous compound becomes a xenobiotic when it is introduced from outside or used in a way that the body treats as a foreign substance. Thyroxine fits this idea: it is normally produced by the thyroid as a hormone, but when given as a medication, it is an exogenous substance that the body processes like any other xenobiotic—absorbed, distributed, metabolized (including deiodination), and eventually excreted. The other options are standard body constituents, not typically encountered as external foreign compounds in usual pharmacology contexts, so they don’t illustrate the concept as clearly.