Pyrimethamine also inhibits which step of folate synthesis?

Pyrimethamine is an antimalarial and antiparasitic agent that primarily inhibits the enzyme dihydrofolate reductase (DHFR) in the folate synthesis pathway. This enzymatic step is crucial as it catalyzes the conversion of dihydrofolate (DHF) to tetrahydrofolate (THF), which is necessary for the synthesis of nucleic acids and amino acids in many organisms.

In the context of the stages of folate synthesis, the inhibition caused by pyrimethamine specifically targets this second step of the pathway, making it essential for limiting the availability of tetrahydrofolate. This mechanism is particularly significant since many rapidly dividing cells, such as those in certain infections, rely heavily on THF for the synthesis of DNA and RNA. Thus, by inhibiting this step, pyrimethamine interferes with the growth and reproduction of susceptible organisms, effectively contributing to its therapeutic effects against parasitic infections.

This targeted action is why the correct answer identifies the second step in the folate synthesis pathway as the site of inhibition by pyrimethamine.