Which statement best defines selective toxicity and provides an antibiotic example?

• A. A drug that harms the host more than pathogens; e.g., penicillin disrupts human cell membranes.
• B. A drug that harms pathogens more than the host; e.g., penicillin targets bacterial cell wall synthesis with minimal effect on human cells.
• C. A drug that treats viral infections only; e.g., acyclovir targets viruses.
• D. A drug that kills all cells equally; e.g., chloramphenicol.

Answer: (B)

Selective toxicity means a drug is more harmful to the pathogen than to the host, by targeting something essential to the microbe that humans don’t share. Penicillin is a classic example because it blocks bacterial cell wall synthesis, affecting bacteria’s peptidoglycan layer while human cells lack cell walls. This differential target allows the antibiotic to kill or inhibit bacteria with relatively low toxicity to the person.

The other statements miss the idea: one suggests the drug harms the host more than the pathogen and misstates penicillin’s action on human membranes; another describes antivirals against viruses rather than a bacterial target; another implies a drug that kills all cells equally, which would be non-selective toxicity.