**Core Concept**
DNA-dependent RNA synthesis refers to transcription, the process where RNA is synthesized using DNA as a template. Inhibitors of this process interfere with RNA polymerase activity, disrupting gene expression.

**Why the Correct Answer is Right**
Rifampicin specifically inhibits bacterial RNA polymerase by binding to the beta subunit of the enzyme, preventing the initiation of RNA synthesis. This results in a potent inhibition of DNA-dependent RNA synthesis, making it a key antibiotic in treating bacterial infections like tuberculosis. Its mechanism is highly selective and does not affect eukaryotic RNA polymerases.

**Why Each Wrong Option is Incorrect**
Option B: Ethambutol inhibits arabinosyl transferase in mycobacterial cell wall synthesis, not RNA polymerase. It affects cell wall integrity, not transcription.
Option C: Colchicine inhibits microtubule assembly, disrupting mitosis and cellular transport, but does not interfere with RNA synthesis.
Option D: Chloromycetin (chloramphenicol) inhibits bacterial protein synthesis by binding to the 50S ribosomal subunit, not RNA polymerase or transcription.

**Clinical Pearl / High-Yield Fact**
Rifampicin is the only option among these that directly targets RNA polymerase. It is a first-line drug for tuberculosis and has a high risk of drug interactions due to its effect on cytochrome P450 enzymes.

✓ Correct Answer: A. Rifampicin