The excretion of weakly basic drugs will be more rapid in acidic urine than in basic urine because:

• A. All drugs are more rapidly excreted in acidic urine
• B. The drug will exist primarily in the unionized form, which cannot be reabsorbed
• C. Weak bases cannot be reabsorbed from the kidney tubules
• D. The drug will exist primarily in the ionized form, which cannot be easily reabsorbed

Answer: (B)

The key idea is how drug ionization changes with urine pH and how that affects reabsorption in the kidney. Weak bases become protonated in acidic urine, turning into charged ions. Ionized (charged) forms do not readily cross the renal tubular epithelium back into the bloodstream, so they stay in the urine and are excreted rather than reabsorbed. In contrast, when urine is basic, the weak base remains mainly unionized and can be reabsorbed, so excretion slows down.

So, excretion of a weakly basic drug is faster in acidic urine because it shifts to the ionized form, which cannot be easily reabsorbed. Remember that the unionized form is lipid-soluble and is the form that can be reabsorbed, whereas the ionized form is trapped in the urine.