Fluoroquinolones act on –
**Core Concept**
Fluoroquinolones are a class of broad-spectrum antibiotics that inhibit bacterial DNA replication and transcription. They target the bacterial enzyme **DNA gyrase** and **topoisomerase IV**, which are essential for the bacterial cell's survival.
**Why the Correct Answer is Right**
Fluoroquinolones bind to the active site of DNA gyrase and topoisomerase IV, preventing the enzymes from unwinding and relaxing supercoiled DNA. This leads to an accumulation of supercoiled DNA, which ultimately results in bacterial cell death. The inhibition of these enzymes is specific to bacterial DNA replication and transcription, making fluoroquinolones effective against a wide range of bacterial infections. The mechanism of action of fluoroquinolones is similar to that of coumarin derivatives, which inhibit **human topoisomerase II**.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because fluoroquinolones do not act on bacterial cell membranes. They do not bind to membrane-bound enzymes or disrupt the bacterial cell membrane.
**Option B:** This option is incorrect because fluoroquinolones do not inhibit the bacterial enzyme **RNA polymerase**. While RNA polymerase is essential for bacterial transcription, fluoroquinolones target DNA replication and transcription by inhibiting DNA gyrase and topoisomerase IV.
**Option C:** This option is incorrect because fluoroquinolones do not act on the bacterial cell wall. They do not inhibit the synthesis of peptidoglycan, which is essential for bacterial cell wall formation.
**Clinical Pearl / High-Yield Fact**
When prescribing fluoroquinolones, it's essential to consider the risk of tendon rupture, especially in patients with a history of tendon disorders or those taking corticosteroids. This risk is dose-dependent and increases with the duration of therapy.
**Correct Answer: C. DNA gyrase and topoisomerase IV**