WBR0463

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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