Which of the following blocks replication without getting involved in the DNA strand?

Nalidixic acid: It is active against gram-negative bacteria, especially coliforms: E. coli, Proteus, Klebsiella, Enterobacter, Shigella but not Pseudomonas. It acts by inhibiting bacterial DNA gyrase and is bactericidal. Resistance to nalidixic acid develops rather rapidly. Nalidixic acid is absorbed orally, highly plasma protein bound and paly metabolized in liver: one of the metabolites is active. It is excreted in urine with a plasma tllz -8 hrs. Concentration of the free drug in plasma and most tissues attained with the usual doses is nontherapeutic for systemic infections (MIC values for most susceptible bacteria just approach the &;break-point&; concentration). However, high concentration attained in urine (20-50 times that in plasma) is lethal to the common urinary pathogens. ESSENTIALS OF MEDICAL PHARMACOLOGY K.D.TRIPATHI SIXTH EDITION PAGE NO:687