Final answer:
Sulfonamides are structurally similar to PABA, competitively inhibiting the bacterial enzyme required for folic acid synthesis, distinguishing them as selectively toxic to bacteria.
Step-by-step explanation:
Sulfonamides are similar in structure to para-aminobenzoic acid (PABA). These drugs are structural analogs and competitive antagonists of PABA, which is essential for the synthesis of folic acid in bacteria. Sulfonamides work by inhibiting the enzyme sulfihydropteroate synthase, thereby preventing the conversion of PABA into dihydrofolic acid, which is the precursor of folic acid. Since bacteria need to synthesize their own folic acid, whereas humans obtain it from dietary sources, this difference allows sulfonamides to be selectively toxic to bacteria without affecting human cells.