**Question:** Drug which acts by binding to B site in prokaryotes:

A. Streptomycin
B. Colistin
C. Triclosan
D. Gentamicin

**Core Concept:**
The question is asking about a type of antibiotics that target bacterial cells by binding to a specific site. In the context of prokaryotes (bacteria), the antibiotic can either inhibit protein synthesis, carbohydrate synthesis, or nucleic acid synthesis. Bacterial cell walls and ribosomes are key components involved in these processes.

**Why the Correct Answer is Right:**
The correct answer, B. Colistin, is an antibiotic that works by binding to the B site (B-site) of the bacterial ribosomal large subunit (50S) in prokaryotic cells. This binding prevents the binding of fMet-tRNA to the 50S ribosomal subunit, thereby inhibiting the initiation of protein synthesis and ultimately leading to bacterial cell death. Colistin is considered a last-resort antibiotic due to its selective action on gram-negative bacteria and its minimal impact on eukaryotic cells.

**Why Each Wrong Option is Incorrect:**

A. Streptomycin: This antibiotic works by binding to the 16S rRNA of the 30S small ribosomal subunit, preventing the formation of 70S initiation complex during protein synthesis. Streptomycin is effective against both gram-positive and gram-negative bacteria but has a higher affinity for prokaryotic ribosomes compared to eukaryotic ribosomes.

B. Colistin, as explained, inhibits protein synthesis in prokaryotic cells, while eukaryotic cells are less affected.

C. Triclosan is an antimicrobial agent used as a bacteriostatic agent against gram-positive bacteria. It inhibits bacterial cell wall synthesis by binding to the enzyme enolase, which is involved in the synthesis of peptidoglycan, a crucial component of bacterial cell walls.

D. Gentamicin is an aminoglycoside antibiotic that binds to the 16S rRNA of the 30S ribosomal subunit, preventing peptide bond formation during protein synthesis in prokaryotic cells.

**Clinical Pearl:**
Colistin is a crucial antibiotic used in the treatment of MDR (Multiple Drug Resistance) gram-negative infections, including carbapenem-resistant Enterobacteriaceae (CRE) and Pseudomonas aeruginosa infections. However, its use is limited due to nephrotoxicity and neurotoxicity. The development of resistance against colistin is also a concern due to the widespread use of this antibiotic.