A 2-year-old boy is seen in the emergency department for refusal to walk. His family returned 3 days ago from a weekend camping trip in Arizona. The parents do not recall the boy having any injuries, but he did have a cough and runny nose just before the trip. Since returning home, the parents have noticed the patient limping mildly when running. After day care yesterday, his left knee began hurting. The parents placed a cold compress on the knee last evening, which helped. This morning, the patient refused to walk and cried when the parents took him out of bed. He has no chronic medical conditions and takes no medications. Immunizations are up to date. Temperature is 38.3 C (101 F). The skin over the left knee is warm and edematous. There are no abrasions. The patient resists all attempts at passive range of motion. The knee is aspirated, and synovial fluid reveals a leukocyte count of 53,000/mm³; the joint aspiration specimen is sent for culture. The patient is admitted to receive intravenous vancomycin. After 2 days, he continues to have fever and refuses to walk. Which of the following is the most appropriate to add to this patient's regimen?

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This patient's acute left knee pain, warmth, erythema, restricted range of motion, and synovial fluid leukocyte count of >50,000/mm³ indicate septic arthritis. Most cases are caused by hematogenous dissemination of Staphylococcus aureus (or other gram-positive organisms) and are adequately treated with vancomycin, a bactericidal antibiotic with activity primarily against gram-positive cocci (via inhibition of cell wall cross-linking). However, patients occasionally have continued symptoms despite a few days of vancomycin treatment, which usually indicates infection with an organism not covered by vancomycin (eg, gram-negative bacteria). These patients generally require a broadening of antimicrobial therapy to cover common gram-negative joint pathogens such as Kingella kingae. A third-generation cephalosporin such as ceftriaxone, which has excellent activity against both gram-positive and gram-negative aerobic organisms (and some anaerobic ones), would likely be initiated while culture results are pending. (Choice B) Clindamycin has excellent coverage against Staphylococcus aureus. Although it also treats some anaerobes, it has limited or no activity against aerobic gram-negative pathogens. Therefore, adding clindamycin would not sufficiently broaden coverage. (Choice C) Doxycycline has fairly good activity against gram-positive, gram-negative, and many atypical organisms (eg, Lyme disease). However, it is a bacteriostatic antibiotic; therefore, it is not often used to broaden antimicrobial coverage for severe infections when other antibiotic choices are available. Although doxycycline can be used to treat Lyme disease, Lyme is uncommon in Arizona, and Lyme arthritis is generally a late manifestation (after months or years) of the disorder. (Choice D) Itraconazole treats coccidiodiomycosis, which is endemic in Arizona. However, fungal arthritis is quite uncommon in children and usually arises in the setting of concomitant fungal lung disease and/or immunocompromise. In addition, it generally manifests weeks (not days) after inoculation. (Choice E) Metronidazole treats most gram-negative and gram-positive anaerobic bacteria but has limited activity against aerobes. Because anaerobes do not often cause joint infection, metronidazole is not frequently utilized to broaden coverage unless a risk factor (eg, tooth abscess) for anaerobic infection is present. Educational objective: Septic arthritis is usually caused by gram-positive aerobic bacteria, particularly Staphylococcus aureus. Therefore, empiric treatment with vancomycin is generally adequate. However, a minority of patients have continued symptoms despite a few days of vancomycin treatment, which usually indicates infection with a gram-negative bacterium. These patients require the addition of an antibiotic that covers aerobic gram-negative pathogens, such as a third-generation cephalosporin (eg, ceftriaxone).

A 2-Year-Old Boy Is Seen in the Emergency Department for Refusal