Which of the following is classified as a narrow-spectrum antibiotic?

Isoniazid is classified as a narrow-spectrum antibiotic because it is specifically effective against Mycobacterium tuberculosis, the bacteria that causes tuberculosis. This targeted action makes it particularly useful in the treatment of this specific infection, while having minimal efficacy against other types of bacteria. As a narrow-spectrum agent, isoniazid is ideal for situations where the infection is known to be due to this particular organism, reducing the risk of collateral damage to the normal flora and limiting the development of antibiotic resistance.

In contrast, ceftriaxone, ciprofloxacin, and imipenem are broad-spectrum antibiotics, meaning they act against a wide variety of bacteria, making them useful for treating infections when the specific causative organism is not yet identified. Broad-spectrum antibiotics can affect both harmful bacteria and beneficial bacteria, whereas narrow-spectrum agents like isoniazid target specific pathogens. This specificity carries significant clinical implications for treatment strategies and resistance management.