**Question:** Not true about vancomycin is:

A. Vancomycin is effective against gram-negative bacteria.
B. Vancomycin is not absorbed systemically after intravenous administration.
C. Vancomycin is a substrate for the drug-transporting enzyme P-glycoprotein.
D. Vancomycin primarily damages bacterial cell walls by inhibiting protein synthesis.

**Core Concept:** Vancomycin is a glycopeptide antibiotic used to treat infections caused by Gram-positive bacteria, particularly those resistant to other antibiotics. It works by inhibiting bacterial cell wall synthesis, causing cell lysis and death.

**Why the Correct Answer is Right:**
Vancomycin is effective against gram-positive bacteria because it targets the synthesis of peptidoglycan, a crucial component of the bacterial cell wall. Gram-negative bacteria have a different cell wall structure and are generally less susceptible to vancomycin.

**Why Each Wrong Option is Incorrect:**
A. Vancomycin is not effective against gram-negative bacteria due to the differences in their cell wall structure, which makes it less accessible to vancomycin.
B. Vancomycin is absorbed systemically via passive diffusion, and while it can reach concentrations in the blood, it is not specifically designed for systemic absorption.
C. Vancomycin is not a substrate for P-glycoprotein (a drug-transporting enzyme) because its mechanism of action, targeting cell wall synthesis, is different from the typical substrates for P-glycoprotein.
D. Vancomycin primarily damages bacterial cell walls by inhibiting protein synthesis, which is incorrect. Vancomycin inhibits the synthesis of peptidoglycan, not protein synthesis.

**Clinical Pearl:**
Understanding the differences in antibiotic susceptibility between gram-positive and gram-negative bacteria is crucial for effective antimicrobial therapy. Vancomycin's mechanism of action is crucial in distinguishing it from other antibiotics.