Which condition would prevent a drug from being absorbed to its maximum due to efflux?

• A. High intestinal blood flow
• B. Low molecular weight
• C. Presence of p-glycoprotein
• D. High dose

Answer: (C)

Efflux transporters like P-glycoprotein on the intestinal lining actively pump drugs back into the gut lumen. This ATP-powered action lowers the amount of drug that can cross the enterocyte and reach the bloodstream, preventing the drug from achieving its maximum absorption. When a drug is a P-gp substrate, oral bioavailability can be significantly reduced because part of what enters the intestinal cells is exported back out before it can be absorbed systemically.

Other factors listed wouldn’t directly cause efflux-related limitation of maximum absorption: high intestinal blood flow can help move absorbed drug away from the site but doesn’t cause active efflux; a low molecular weight generally aids passive diffusion into the blood rather than being expelled; a high dose might overwhelm some transporters, but the defining mechanism that reduces maximum absorption here is the presence of an active efflux transporter like P-glycoprotein.