What step in the folic acid synthesis process does trimethoprim block?

• A. The fourth step, which prevents the formation of tetrahydrofolic acid from dihydrofolic acid
• B. The second step
• C. The first step
• D. The third step, which prevents the formation of tetrahydrofolic acid from dihydrofolic acid

Answer: (D)

Trimethoprim blocks the reduction step that converts dihydrofolic acid into tetrahydrofolic acid by inhibiting the bacterial enzyme dihydrofolate reductase. Without tetrahydrofolate, essential one-carbon transfers required to synthesize thymidine (a DNA building block) and purines can’t proceed, so DNA synthesis stalls and bacterial growth is halted. Sulfamethoxazole acts earlier in the pathway to prevent dihydropteroate formation, but trimethoprim’s action is specifically at the DHFR-catalyzed reduction, making this the correct description of its target. This selectivity helps explain why the drug is effective against bacteria while reducing effects on human cells, since human cells rely less exclusively on this bacterial enzyme.