A 26-year-old woman in her first trimester of pregnancy is evaluated in the emergency department for an acutely painful left wrist and painful, swollen right knee. Four days ago, she developed flu-like symptoms characterized by migratory joint pains. Her pregnancy had been uncomplicated. On physical examination, the right knee has a warm effusion, and the left wrist is slightly warm but not swollen, Passive wrist motion is uncomfortable and resisted extension is extremely painful. The joint capsule of the knee is tender. On pelvic examination, there is a scant mucoid cervical discharge. The remainder of the general examination is normal. Arthrocentesis of the knee yields 20 mL of fluid (leukocyte count, 14,000/14L; BOX neutrophils), Gram stain is negative Which of the following is the most appropriate next step in management?