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A 13-year-old boy presents to the emergency department in Rhode Island with knee pain for three days duration. It was atraumatic in onset. He has been afebrile. Upon physical examination, he is noted to have a tense, swollen knee and the skin lesion seen in Figure A. An oral course of antibiotics is selected as definitive management. What antibiotic would be most appropriate for this patient, and what is its primary mechanism of action?
Doxycycline, Inhibits the 50s ribosomal subunit
Doxycycline, Inhibits the 30s ribosomal subunit
Cephalexin, Blocks cell wall synthesis
Cephalexin, Folic acid inhibitor
Rifampin, RNA synthesis inhibitor
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Doxycycline is a first-line treatment for Lyme disease arthritis. It works to inhibit protein synthesis by binding to the 30s ribosomal subunit. It also binds to the 50s ribosomal subunit, but to a much lesser extent.
The differentiation between Lyme arthritis and septic arthritis is often very difficult and has important treatment implications. In patients with Lyme disease arthritis and no carditis or meningoencephalitis, oral antibiotics are the first-line treatment. Either doxycycline, amoxicillin, or cefuroxime may be used for a 28-day course. However, if septic arthritis is diagnosed, immediate irrigation and debridement is advised.
Smith et al. review the orthopaedic implications of Lyme disease. They state that Lyme arthritis should be treated with a 28-day course of doxycycline or amoxicillin for 28 days. If therapy fails, IV antibiotics or arthroscopic synovectomy may be considered.
Willis et al. review 10 cases of acute arthritis presenting as septic arthritis that were eventually diagnosed with Lyme disease. They recommend the Rapid 1-hour Lyme EIA in areas where Lyme disease is prevalent to avoid unnecessary irrigation and debridement.
Milewski et al. review a cohort of patients with either Lyme arthritis or septic arthritis of the knee. They find that children with septic arthritis are more likely to have a higher peripheral white blood-cell count, a higher joint fluid cell count, are more likely to be febrile, and refuse to bear weight more commonly.
Figure A shows a target lesion, which is characteristic of Lyme disease. Illustration A shows the mechanism of action of various commonly used antibiotics. Illustration B shows the specific method of action of tetracyclines.
Answer 1: Macrolides, chloramphenicol, clindamycin, linezolid, and streptogramins inhibit the 50s ribosomal subunit.
Answer 3: Penicillins, cephalosporins, vancomycin, beta-lactamase inhibitors, carbapenems, aztreonam, polymycin, and bacitracin inhibit cell wall synthesis.
Answer 4: Sulfonamides and trimethoprim are folic acid inhibitors.
Answer 5: Rifampin is a RNA synthesis inhibitor.
Smith BG, Cruz AI Jr, Milewski MD, Shapiro ED.
J Am Acad Orthop Surg. 2011 Feb;19(2):91-100. PMID: 21292932 (Link to Abstract)
Smith, JAAOS 2011
Willis AA, Widmann RF, Flynn JM, Green DW, Onel KB
J Pediatr Orthop. 23(1):114-8. PMID: 12499956 (Link to Abstract)
Willis, JPO 2003
Milewski MD, Cruz AI, Miller CP, Peterson AT, Smith BG
J Bone Joint Surg Am. 2011 Feb;93(3):252-60. PMID: 21266639 (Link to Abstract)
Milewski, JBJS 2011
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