A 35-year-old man is evaluated for a 2-month history of abrupt left knee swelling. He notes prominent stiffness of both joints but no significant pain. He previously felt well. He lives in Vermont and goes hiking during the summer. He has not had any episodes of diarrhea or abdominal pain and reports no trauma to the knee, fever, rash, or known insect bites. He does not have a history of sexually transmitted infections. He has no history of injection drug use and does not take any medications.
On physical examination, temperature is 37.1 °C (98.8 °F), blood pressure is 115/70 mm Hg, pulse rate is 82/min, and respiration rate is 12/min. BMI is 20. There is a large effusion over the left knee with warmth and mild tenderness but no overlying erythema; range of motion is limited by swelling, but stability is intact. There is no heart murmur. Lung and abdominal examinations are normal. There are no skin lesions.
Laboratory studies reveal an erythrocyte sedimentation rate of 12 mm/h and a leukocyte count of 6000/µL (6.0 × 109/L).
Radiograph of the left knee shows a large effusion but is otherwise unremarkable.
Which of the following is most likely to provide the diagnosis?