Which of the following antimicrobial drugs is entirely synthetic?

Sulfanilamide is entirely synthetic and represents an important class of antimicrobial drugs known as sulfonamides. These compounds were developed in the laboratory and are designed to mimic para-aminobenzoic acid (PABA), a substrate used by bacteria for the synthesis of folate. By competing with PABA, sulfanilamide effectively inhibits bacterial growth.

In contrast, penicillin, neomycin, and actinomycin are derived from natural sources. Penicillin comes from fungi, neomycin is produced by certain strains of bacteria (specifically, Streptomyces species), and actinomycin is also derived from Streptomyces. Therefore, sulfanilamide stands out as the only option that is wholly synthetic, illustrating the advancements in drug development that have allowed for the creation of effective antimicrobial agents that do not rely on natural sources.