Which antimicrobial medication inhibits nucleic acid synthesis in susceptible pathogens?

The antimicrobial medication that inhibits nucleic acid synthesis in susceptible pathogens is Actinomycin. This drug works by intercalating into DNA, specifically binding to the double-stranded helix, which prevents the normal functioning of RNA polymerase during the transcription process. By disrupting RNA synthesis, Actinomycin effectively halts protein synthesis and cellular growth in bacteria and cancer cells.

In contrast, Penicillin and Cephalosporin are beta-lactam antibiotics that primarily act by inhibiting cell wall synthesis, not nucleic acid synthesis. Erythromycin, an antibiotic in the macrolide class, inhibits protein synthesis by binding to the ribosomal subunit but does not impede nucleic acid synthesis directly. Therefore, Actinomycin is the correct answer as it specifically targets and inhibits nucleic acid synthesis in susceptible pathogens.