Why is absorption usually faster than distribution?

• A. Because Cgi >> Cp so gradient favors entry into blood
• B. Because Cp >> Cgi
• C. Because distribution occurs faster than absorption
• D. Because absorption is saturable only

Answer: (A)

The key idea is that the rate of absorption is driven by the concentration difference between the GI lumen and the blood. Right after an oral dose, there is a high drug concentration in the intestinal lumen (C_gi) and only a small amount in the plasma (C_p). This large gradient pushes the drug across the intestinal wall into the bloodstream by diffusion, making absorption occur rapidly.

Distribution, by contrast, moves drug from the blood into tissues. This step depends on tissue blood flow, membrane permeability, binding, and partitioning, and it doesn’t enjoy as large a driving gradient as absorption does. So it tends to proceed more slowly than absorption.

That’s why the statement that C_gi is much greater than C_p, creating a strong gradient for entry into the blood, best explains why absorption is typically faster.

The other options don’t fit: reversing the gradient (Cp >> Cgi) isn’t the situation right after dosing; saying distribution is faster than absorption contradicts the usual sequence; and claiming absorption is only saturable ignores that absorption can be linear or involve other kinetics depending on the drug.