WBR0507

Author	[[PageAuthor::Serge Korjian M.D., Gonzalo A. Romero, M.D. (Reviewed by Alison Leibowitz)(Reviewed by Serge Korjian)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Embryology, MainCategory::Microbiology, MainCategory::Pharmacology
Sub Category	SubCategory::Infectious Disease
Prompt	[[Prompt::A 2-year-old girl is brought to the pediatrician because her mother is concerned that her teeth have turned a dark shade of gray since her last check-up. The mother explains that her daughter has been developing normally, but she recently started refusing formula. She also reports that approximately 2 months ago, the child had an episode of cough and sore throat for which she gave her some left-over antibiotics she had at home. Physical exam is unremarkable except for annular gray-brown stains apparent mostly across the front teeth. Which of the following was most likely mechanism of action of the drug used in this patient?]]
Answer A	AnswerA::Prevents attachment of aminoacyl-tRNA to 30S ribosomal subunit
Answer A Explanation	AnswerAExp::Tetracycline
Answer B	AnswerB::Inhibits the formation of initiation complex
Answer B Explanation	AnswerBExp::Aminoglycosides
Answer C	AnswerC::Inhibits protein synthesis by binding to the 50S ribosomal subunit
Answer C Explanation	AnswerCExp::Clindamycin
Answer D	AnswerD::Inhibits the translocation by binding to the 23S ribosomal subunit
Answer D Explanation	AnswerDExp::Macrolide
Answer E	AnswerE::Inhibits protein elongation by blocking peptidyl-transferase
Answer E Explanation	AnswerEExp::Chloram
Right Answer	RightAnswer::A
Explanation	[[Explanation::Tetracycline antibiotics are a group of bacteriostatic agents whose name is derived from their main structure comprising four hydrocarbon rings. Tetracycline antibiotics inhibit protein synthesis by preventing the binding of aminoacyl-tRNA to the mRNA-ribosome complex. The main site of action of these agents is the 30S ribosomal subunit. Tetracyclines have a broad spectrum of activity against both gram-positive and gram-negative organisms. Despite growing resistance, tetracyclines still have several indications including atypical (Chlamydia, Legionella), rickettsial (typhus, ehrlichiosis, anaplasmosis, rocky mountain spotted fever), and spirochetal (lyme disease, leptospirosis, syphylis) infections. Administration of these agents should be separated from calcium or iron containing preparations as well as dairy products because cations bind to tetracycline molecules and inhibit proper absorption in the gut. The main mechanism of resistance to tetracyclines is the acquisition of an efflux protein transporter through a resistance plasmid. Another mechanism is through acquiring a protein that blocks the binding of the tetracycline to the ribosomal subunit. Major side effects of tetracyclines include photo-sensitivity, discoloration of the teeth, and rarely pseudotumor cerebri with chronic administration. Tetracycline administration is contraindicated in pregnant and lactating women, and in children <8 years of age because use is associated with abnormal teeth and bone development as well as permanent staining of the teeth. Teeth stains are typically annular, may be more pronounced either at the crown or the root and fade as they move to the opposite aspect of the tooth. Educational Objective: References: ]]
Approved	Approved::Yes
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