What does every step of a drug's journey through the body involve?

• A. Crossing a membrane
• B. Binding to a receptor
• C. Metabolism
• D. Excretion

Answer: (A)

Movement across biological membranes is required in every leg of a drug’s journey through the body. To enter the body from the gut, a drug must cross the intestinal lining. Once in the bloodstream, it must cross capillary and tissue cell membranes to reach various tissues. Inside cells, metabolism typically happens at intracellular enzymes, so the drug has to cross the cell membrane to get to those enzymes. Finally, excretion usually involves moving the drug or its metabolites across renal or biliary membranes to be eliminated in urine or bile. While receptor binding or other interactions can occur at different stages, the universal thread binding all steps is membrane transport—drugs moving from one compartment to another by crossing membranes.