Which of the following antimicrobial stops protein synthesis by inhibition of translocation?
**Core Concept**
The question is testing the mechanism of action of antimicrobials that target protein synthesis in bacteria. Specifically, it is asking about the step of translocation, which is a critical phase of protein synthesis where the ribosome moves along the mRNA to facilitate the transfer of amino acids to the growing polypeptide chain.
**Why the Correct Answer is Right**
The correct answer is an antimicrobial that inhibits translocation by binding to the 50S subunit of the bacterial ribosome. This binding blocks the movement of the ribosome along the mRNA, preventing the transfer of amino acids to the growing polypeptide chain. As a result, protein synthesis is terminated, and the bacterial cell dies. This mechanism is unique to this class of antimicrobials and is a key point of differentiation from other antimicrobials that target different steps of protein synthesis.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it targets the initiation phase of protein synthesis, rather than translocation. It binds to the 30S subunit of the ribosome, preventing the assembly of the initiation complex.
**Option B:** This option is incorrect because it targets the elongation phase of protein synthesis, but not specifically translocation. It binds to the 50S subunit of the ribosome, but at a different site than the correct answer.
**Option C:** This option is incorrect because it targets a different cellular process altogether, rather than protein synthesis.
**Clinical Pearl / High-Yield Fact**
It's worth noting that this class of antimicrobials is effective against a wide range of bacteria, including those that are resistant to other types of antimicrobials. However, resistance to this class of antimicrobials is also a growing concern, and careful use and monitoring are necessary to prevent the development of resistance.
**Correct Answer:** C. Macrolides, such as erythromycin, inhibit protein synthesis by binding to the 50S subunit of the bacterial ribosome, specifically blocking the translocation step.