A 70-year-old man complains of fever and pain in his left knee. Several days previously, he suffered an abrasion of his knee while working in his garage. The knee is red, warm, and swollen. An arthrocentesis is performed, which shows 200,000 leukocytes/µL and a glucose of 20 mg/dL. No crystals are noted. Which of the following is the most important next step?
A. Gram stain and culture of joint fluidThe clinical and laboratory picture suggests an acute septic arthritis. The most important first step is to determine the etiologic agent of the infection. Staphylococcus aureus is the most likely agent in this setting, and antibiotics with potent anti-Staph effect are usually started empirically while awaiting the culture results. Synovial leukocyte counts in gout typically range between 2000/µL and 50,000/µL; in addition, serum uric acid levels are often normal in acute gout. In the absence of negatively birefringent crystals in the synovial fluid, a uric acid level will not be helpful. There are no symptoms suggesting connective tissue disease. Gonococci can cause a septic arthritis, but a urethral culture in the absence of urethral discharge would not be helpful. Antineutrophil cytoplasmic antibodies are present in certain vasculitides. There is no indication of systemic vasculitis in this patient.