Which chemotherapeutic class inhibits bacterial folic acid synthesis by competitive inhibition with p-aminobenzoic acid?

• A. Penicillins
• B. Macrolides
• C. Sulfonamides
• D. Aminoglycosides

Answer: (C)

A drug that inhibits bacterial folic acid synthesis by competing with p-aminobenzoic acid acts as a structural mimic of PABA and blocks the enzyme dihydropteroate synthase. By occupying this enzyme’s active site, it prevents the formation of dihydropteroate and thus halts the production of tetrahydrofolate, a carrier of one-carbon units necessary for making thymidine and purines. Without folate-derived cofactors, bacterial DNA synthesis cannot proceed, leading to growth arrest. Humans don’t synthesize folate in this way and obtain it from the diet, so this mechanism selectively targets bacteria rather than human cells. This disruptive effect on the folate pathway is characteristic of sulfonamides, not the other classes listed, which affect cell wall synthesis or protein synthesis instead.