Which statement best describes tissue permeability effects on distribution rate?

• A. Lipophilic drugs demonstrate slower tissue penetration.
• B. Lipophilic drugs demonstrate faster tissue penetration due to higher membrane permeability.
• C. Membrane permeability does not affect distribution.
• D. Permeability only matters for organs with high perfusion.

Answer: (B)

The main idea is that how fast a drug distributes into tissues depends on its ability to cross cell membranes. Lipophilic drugs cross lipid membranes more easily, so they penetrate tissues faster and reach distribution sites sooner. While blood flow to a tissue supplies the drug, the rate at which it moves from blood into the tissue rests largely on membrane permeability. This is why lipophilic drugs tend to have a faster distribution rate than hydrophilic ones.

The other statements don’t fit because: lipophilic drugs do not demonstrate slower penetration; membrane permeability absolutely affects distribution (it's a key determinant of rate, not a negligible factor); and permeability matters across different tissues, not only those with high perfusion.