Which statement best describes a narrow-spectrum antibiotic agent?

Which statement best describes a narrow-spectrum antibiotic agent?

• A. An antimicrobial that is effective only against bacteria
• B. An antimicrobial that has a narrow therapeutic index
• C. An antimicrobial that can only be given in very small doses because of toxicity concerns
• D. An antimicrobial that is mainly effective against a few Gram-positive or Gram-negative bacteria

Answer: (D)

A narrow-spectrum antibiotic targets a small group of bacteria, often just a few Gram-positive or a few Gram-negative species. This makes it ideal when the causative organism is known, as it treats the infection while sparing most other bacteria and helping to limit resistance. The description that an antimicrobial is mainly effective against a few Gram-positive or Gram-negative bacteria precisely captures this limited range. In contrast, broad-spectrum antibiotics affect a wider variety of bacteria, which is useful when the pathogen isn’t identified but can disrupt normal flora. The other options describe different concepts—therapeutic index relates to safety margins, and dosing concerns refer to toxicity, not the spectrum of activity.

A narrow-spectrum antibiotic targets a small group of bacteria, often just a few Gram-positive or a few Gram-negative species. This makes it ideal when the causative organism is known, as it treats the infection while sparing most other bacteria and helping to limit resistance. The description that an antimicrobial is mainly effective against a few Gram-positive or Gram-negative bacteria precisely captures this limited range. In contrast, broad-spectrum antibiotics affect a wider variety of bacteria, which is useful when the pathogen isn’t identified but can disrupt normal flora. The other options describe different concepts—therapeutic index relates to safety margins, and dosing concerns refer to toxicity, not the spectrum of activity.