USMLE Step 1 Question 356

Question:

A 20-year-old college student returns from a weekend camping trip. One week later, the patient develops an expanding annular rash with central clearing on his ankle and is being treated with doxycycline for 5 weeks. The drug acts by inhibiting which of the following processes?

Correct Answer: C.  Binding of the tRNA to the mRNA-ribosomal complex via the acceptor A site on the mRNA

Doxycycline is a tetracycline. It is a bacteriostatic inhibitor of protein synthesis that acts by reversible binding to 30S ribosomal subunit to inhibit the acceptor side on the mRNA and thus block binding of tRNA to the mRNA-ribosomal complex. Doxycycline is used for the treatment of Stage 1 Lyme Disease.

The Aminoglycosides acts by irreversible binding to the 30S ribosomal subunit to inhibit formation of an active initiation complex. As a result, misreading of the mRNA template occurs.

Erythromycin is a macrolide that acts by reversible binding to the 50S ribosomal subunit of gram-positive microorganisms to inhibit translocation of the peptidyl molecule from A-site to P-site on mRNA.

Clindamycin and the Streptogranins are examples of drugs that bind to the 50S subunit to cause bacteriostatic inhibition of protein synthesis.

Drugs that work by inhibiting the activity of the DNA gyrases are the Quinolones. The result is a bactericidal effect. This class of drugs includes Ciprofloxacin and norfloxacin.