Which drug is active against parasitic infections and has importance for diseases caused by anaerobic bacteria?

• A. Isoniazid
• B. Metronidazole
• C. Rifampin
• D. Trimethoprim

Answer: (B)

Metronidazole is chosen because it is activated specifically in anaerobic organisms to become cytotoxic. In low-oxygen conditions, its nitro group is reduced by anaerobic microbial enzymes to reactive intermediates that damage DNA, causing strand breaks and inhibition of nucleic acid synthesis. This selective activation makes it effective against both protozoan parasites such as Giardia lamblia, Entamoeba histolytica, and Trichomonas vaginalis, and anaerobic bacteria like Bacteroides and other anaerobic flora. Oxygen presence in aerobic organisms prevents this activation, so they are less susceptible. The other drugs have different targets and spectrums: one targets mycobacterial cell-wall synthesis, another inhibits bacterial RNA synthesis, and another blocks folate synthesis—none are as broadly active against anaerobes and the mentioned parasites.