Passive diffusion of substances across the placenta is enhanced by all of the following EXCEPT

Passive diffusion of substances across the placenta is enhanced by all of the following EXCEPT

• A. Decreased maternal protein binding
• B. Low molecular weight of the substance
• C. High water solubility of the substance
• D. Large concentration gradient of the drug

Answer: (C)

Passively crossing the placenta is driven by easy access to the unbound drug, its size, how lipid-soluble it is, and the concentration gradient. Drugs that are less bound to maternal proteins have more unbound fraction to diffuse. Small molecules face less of a size barrier, so they cross more readily. A strong concentration gradient provides the driving force for diffusion. In contrast, highly water-soluble (hydrophilic) drugs don’t cross lipid membranes as readily by passive diffusion; they tend to stay in aqueous compartments or require other transport mechanisms. So, high water solubility does not enhance placental passive diffusion, making that option the exception.

Passively crossing the placenta is driven by easy access to the unbound drug, its size, how lipid-soluble it is, and the concentration gradient. Drugs that are less bound to maternal proteins have more unbound fraction to diffuse. Small molecules face less of a size barrier, so they cross more readily. A strong concentration gradient provides the driving force for diffusion. In contrast, highly water-soluble (hydrophilic) drugs don’t cross lipid membranes as readily by passive diffusion; they tend to stay in aqueous compartments or require other transport mechanisms. So, high water solubility does not enhance placental passive diffusion, making that option the exception.