Clinical therapeutics: Which of the following is NOT useful for the treatment of viral infections (select the agent that is ineffective against viruses)?

Introduction / Context:
The question distinguishes antiviral strategies from antibacterial agents. Effective management of viral diseases relies on antivirals, immune modulators, and passive or active immunization—not on antibiotics that target bacterial structures such as cell walls or ribosomes.

Given Data / Assumptions:

• Acyclovir is a nucleoside analog used against herpesviruses.
• Interferon refers to cytokines that modulate antiviral defenses.
• Antibodies (e.g., monoclonal antibodies or convalescent plasma) can neutralize viruses.
• Penicillin targets bacterial peptidoglycan synthesis, which viruses lack.

Concept / Approach:
Viruses use host-cell machinery and have no cell wall or independent metabolism. Antibiotics like penicillin have no viral targets. In contrast, acyclovir inhibits viral DNA polymerase after phosphorylation by viral thymidine kinase, interferon enhances innate antiviral responses, and antibodies can bind and neutralize virions or block entry.

Step-by-Step Solution:

Verification / Alternative check:
Clinical guidelines reserve antibiotics for bacterial coinfections; overuse in viral illness risks antimicrobial resistance without therapeutic benefit against the virus itself.

Why Other Options Are Wrong:

• Acyclovir: active against herpes simplex and varicella-zoster; clearly antiviral.
• Interferon: used in certain chronic viral infections and for immune modulation.
• Antibodies: neutralizing monoclonal antibodies are established antivirals in several diseases.

Common Pitfalls:
Assuming “antibiotic” is a synonym for “antimicrobial”; antibiotics are a subset targeting bacteria, not viruses.

Final Answer:
Penicillin