WBR0463

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Author [[PageAuthor::Rim Halaby, M.D. [1], Alison Leibowitz [2] (Reviewed by Alison Leibowitz)]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Microbiology
Sub Category SubCategory::Dermatology, SubCategory::General Principles, SubCategory::Infectious Disease
Prompt [[Prompt::A 37-year-old female presents to the physician’s office with a fever of two days duration. Upon further questioning, the patient informs you she went camping on the East Coast a week ago and was bitten by ticks. On physical examination, you note a rash on her left sole and administer her an antibiotic regimen. Which of the following mechanisms of action is most likely associated with the prescribed antibiotic?]]
Answer A AnswerA::A bacteriostatic antibiotic that inhibits the attachment of aminoacyl t-RNA by binding to 30S ribosomal subunit
Answer A Explanation AnswerAExp::See explanation.
Answer B AnswerB::A bacteriostatic antibiotic that inhibits the translocaton during protein synthesis by binding to 50S ribosomal subunit
Answer B Explanation [[AnswerBExp::Macrolides, a bacteriostatic class of antibiotics, inhibit translocaton during protein synthesis by binding to the 23S rRNA of the 50S ribosomal subunit. Macrolides are not typically used to treat rickettsial infections.]]
Answer C AnswerC::A bactericidal antibiotic that inhibits the attachment of aminoacyl t-RNA by binding to 50S ribosomal subunit
Answer C Explanation [[AnswerCExp::Tetracyclines do not act on the 50S ribosome, regardless of whether they are bactericidal or bacteriostatic.]]
Answer D AnswerD::A bactericidal antibiotic that inhibits the translocation during protein synthesis by binding to 50S ribosomal subunit
Answer D Explanation [[AnswerDExp::Macrolides are bacteriostatic and are not typically used to treat rickettsial infections.]]
Answer E AnswerE::A bactericidal antibiotic that activates the attachment of aminoacyl-tRNA by binding to 30S ribosomal subunit
Answer E Explanation AnswerEExp::Mediation of aminoacyl t-RNA attachment is not the mechanism of action for antibiotics.
Right Answer RightAnswer::A
Explanation [[Explanation::The patient most likely presents with Rocky Mountain spotted fever (RMSF), caused by Rickettsia rickettsii and contracted with a tick bite. RMSF frequently manifests with a migrating rash of the the palms and soles, headache, and fever.

Doxycycline is usually administered to patients with rickettsial infections. The mechanism of action of Doxycycline, a tetracycline (baceteriostatic antibiotic, is inhibiting the attachment of aminoacyl-tRNA, by binding to the 30S ribosomal subunit.
Educational Objective: Rocky Mountain spotted fever (RMSF), caused by Rickettsia rickettsii and contracted with a tick bite manifests with a migrating rash of the the palms and soles, headache, and fever. Doxycyline, a tetracycline, inhibits the attachment of aminoacyl-tRNA by binding to 30S ribosomal subunit, and is frequently administered to patients with rickettsial infections.
References: First Aid 2014 page 144]]

Approved Approved::Yes
Keyword WBRKeyword::Rocky Mountain spotted fever, WBRKeyword::RMSF, WBRKeyword::doxycycline, WBRKeyword::tetracycline, WBRKeyword::antibiotic, WBRKeyword::tick, WBRKeyword::mechanism of action, WBRKeyword::rash, WBRKeyword::ribosomal, WBRKeyword::microbiology
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