Question

5-fluorouracil is absorbed by

• A. Passive diffusion
• B. Active transport
• C. Pinocytosis
• D. Ion pair absorption

Hint:

For many lipophilic drugs like 5-fluorouracil, passive diffusion is the key absorption mechanism due to the concentration gradient.

Answer 1

The Correct Option is A

5-fluorouracil is a small molecule with sufficient lipid solubility to cross the gastrointestinal membrane simply by moving from a region of higher concentration (the gut lumen) to a region of lower concentration (the blood), without needing a carrier protein or energy input. This passive movement down the concentration gradient is what defines passive diffusion, distinguishing it from active transport (which needs a carrier and energy), pinocytosis (relevant to larger particles), and ion pair absorption (relevant to strongly ionized drugs).

Therefore, the correct answer is Passive diffusion.

Answer 2

Another way to work through this is by asking what conditions would be needed for each mechanism to apply, and checking whether 5-fluorouracil meets them.

1. Passive diffusion: This mechanism only requires a concentration gradient and a membrane-permeable molecule, both of which are satisfied by 5-fluorouracil's small size and moderate lipophilicity.
2. Active transport: This would require 5-fluorouracil to be recognized by a specific transporter protein and for the cell to expend metabolic energy, which is not how this drug is typically absorbed.
3. Pinocytosis: This mechanism is reserved for larger molecules or fluid droplets engulfed by the cell membrane, a condition that does not apply to a small molecule like 5-fluorouracil.
4. Ion pair absorption: This requires the drug to exist predominantly in an ionized state and pair with an oppositely charged counter-ion, which is not the defining absorption pathway for this compound.

Only the conditions for passive diffusion are satisfied by 5-fluorouracil's properties.

Therefore, the correct answer is Passive diffusion.