A 70-year-old, sexually active male complains of fever and pain in his left knee. Several days ago, the patient skinned his knee while working in his garage. The knee is red, warm, and swollen. An ahocentesis is performed, which shows 200,000 leukocytes/uL and a glucose of 20 mg/dL. No crystals are noted. The most impoant next step is

(Stobo, 23/e, pp 251-256.) The clinical and laboratory picture suggests an acute septic ahritis. The most impoant first step is to determine the etiologic agent of the infection. Gout would be unlikely to produce the 200,000 leukocytes in the joint fluid. There is no history of symptoms suggesting connective tissue disease. Gonococci can cause a septic ahritis, but a urethral culture in the absence of rethral discharge would not be helpful.