Which drug class decreases the synthesis of thymidylate and purine nucleotides?

Antifolates, such as trimethoprim, act by inhibiting enzymes involved in the folate metabolism pathway, which is essential for nucleotide synthesis. Specifically, trimethoprim inhibits dihydrofolate reductase, an enzyme that catalyzes the reduction of dihydrofolate to tetrahydrofolate. Tetrahydrofolate is a critical cofactor required for the synthesis of thymidylate (a pyrimidine nucleotide) and purines. Therefore, by impeding the production of tetrahydrofolate, antifolates effectively decrease the synthesis of these nucleotides, which are vital for DNA replication and cell division.

Other drug classes listed do not exert this effect. Beta-lactam antibiotics primarily act by inhibiting bacterial cell wall synthesis. HMG-CoA reductase inhibitors, commonly known as statins, reduce cholesterol synthesis and do not influence nucleotide synthesis. Non-selective beta antagonists are primarily used to manage cardiovascular conditions and do not have a mechanism related to nucleic acid metabolism. Thus, antifolates like trimethoprim are the correct choice for decreasing the synthesis of thymidylate and purine nucleotides.