If a drug is not in the stomach and has crossed into plasma due to its charge, what kind of drug can it be?

If a drug is not in the stomach and has crossed into plasma due to its charge, what kind of drug can it be?

• A. Weak base
• B. Ethanol
• C. Weak acid
• D. Neutral compound

Answer: (C)

The important idea is how pH affects whether a drug is ionized or non-ionized, which governs its ability to cross membranes. In the stomach, the environment is very acidic, so a weak acid stays largely non-ionized. Non-ionized molecules cross gastric membranes easily, allowing the drug to move into the bloodstream. Once in plasma, where pH is around 7.4, the weak acid becomes more ionized, which traps it in the blood and reduces its ability to cross membranes again. This pattern points to a weak acid as the type that can cross into plasma due to its charge state changing with pH. A weak base would be protonated and ionized in the stomach, hindering absorption; ethanol and neutral compounds don’t fit the described ionization-driven transfer between stomach and plasma in the same way.

The important idea is how pH affects whether a drug is ionized or non-ionized, which governs its ability to cross membranes. In the stomach, the environment is very acidic, so a weak acid stays largely non-ionized. Non-ionized molecules cross gastric membranes easily, allowing the drug to move into the bloodstream. Once in plasma, where pH is around 7.4, the weak acid becomes more ionized, which traps it in the blood and reduces its ability to cross membranes again. This pattern points to a weak acid as the type that can cross into plasma due to its charge state changing with pH. A weak base would be protonated and ionized in the stomach, hindering absorption; ethanol and neutral compounds don’t fit the described ionization-driven transfer between stomach and plasma in the same way.