WBR0463

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Microbiology
Sub Category	SubCategory::Dermatology, SubCategory::General Principles, SubCategory::Infectious Disease
Prompt	[[Prompt::A 15-year-old girl is brought to the physician’s office with low-grade fever and rash for the past 2 days. Upon further questioning, the patient informs the physician that she went camping on the east coast one week ago and was bitten by ticks. Prior to the onset of fever and rash, she was complaining of nausea, anorexia, and myalgias. On physical examination, the physician notes several blanching maculopapular spots of 1-5 mm on her palms and soles. The physician then prescribes a course of antibiotics to treat the patient's condition. What is the mechanism of action of the most likely prescribed antibiotic agent?]]
Answer A	AnswerA::A bacteriostatic agent that inhibits the attachment of aminoacyl t-RNA by binding to the 30S ribosomal subunit
Answer A Explanation	[[AnswerAExp::Doxycyline is a tetracycline that inhibits the attachment of aminoacyl-tRNA by binding to the 30S ribosomal subunit. Doxycycline is the first line antibiotic therapy for patients with rickettsial infections.]]
Answer B	AnswerB::A bacteriostatic agent that inhibits the translocaton during protein synthesis by binding to the 50S ribosomal subunit
Answer B Explanation	[[AnswerBExp::Macrolides are bacteriostatic antibiotics that inhibit translocation during protein synthesis by binding to the 23S rRNA of the 50S ribosomal subunit. Macrolides are not prescribed to treat rickettsial infections.]]
Answer C	AnswerC::A bactericidal agent that inhibits the attachment of aminoacyl t-RNA by binding to the 50S ribosomal subunit.
Answer C Explanation	[[AnswerCExp::Tetracyclines do not act on the 50S ribosome. Tetracyclines inhibit the attachment of aminoacyl-tRNA by binding to the 30S ribosomal subunit.]]
Answer D	AnswerD::A bactericidal agent that inhibits the translocation during protein synthesis by binding to the 50S ribosomal subunit
Answer D Explanation	[[AnswerDExp::Macrolides are bacteriostatic agents that are not usually prescribed to treat rickettsial infections.]]
Answer E	AnswerE::A bactericidal agent that activates the attachment of aminoacyl-tRNA by binding to 30S ribosomal subunit
Answer E Explanation	AnswerEExp::Activation of the aminoacyl t-RNA attachment is not the mechanism of action of any antibiotic agent.
Right Answer	RightAnswer::A
Explanation	[[Explanation::Rocky mountain spotted fever (RMSF) is caused by Rickettsia rickettsii, a bacteria that is carried by ticks (eg. wood tick or dog tick) and spread to humans by tick bites when ticks attach for more than 20 hours. The incubation period for RMSF is approximately 2-12 days. Despite its name, RMSF is common in eastern USA especially during the spring and summer. The disease is often characterized by a prodrome of non-specific GI symptoms, headaches, and myalgias followed by a fever and a rash. The rash manifests as small maculopapular blanching lesions that initially appear in the palms and soles before they migrate centrally to include the trunk and become petechial, hemorrhagic, and non-blanching. RMSF may be associated with severe neurological complications, including seizures and meningismus, cardiopulmonary disease, and disseminated intravascular coagulopathy (DIC). If left untreated, the rate of death may be as high as 20%. Accordingly, the administration of antibiotics empirically is necessary upon high clinical suspicion even in the absence of the characteristic rash. Work-up results may be delayed up to 2 weeks of infection and may include anemia, leukopenia, thrombocytopenia, hyponatremia with elevated liver enzymes. Although Weil-Felix and complement-fixation reaction assays may be helpful, they are often not sensitive. Accordingly, latex agglutination, hemagglutination, microimmunofluorescence, and ELISA are superior for the diagnosis of RMSF but often unnecessary. Doxycycline is the fist-line antibiotic therapy for RMSF and may be prescribed empirically upon clinical suspicion. Doxycyline inhibits the attachment of aminoacyl-tRNA by binding to the 30S ribosomal subunit. Doxycycline is contraindicated in pregnant women and children < 8 years of age; these patients are often prescribed amoxicillin instead. Educational Objective: Doxycycline is the fist-line antibiotic therapy for RMSF. Doxycyline inhibits the attachment of aminoacyl-tRNA by binding to 30S ribosomal subunit. Doxycycline is contraindicated in pregnant women and children < 8 years of age. References: Abramson JS, Laurence GB. Rocky mountain spotted fever. Ped Infect Dis J. 1999;18(6):539-40. 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::Antibiotics, WBRKeyword::Mechanism of action
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