Question

A $28$-year-old male is seen at the university health clinic for right knee pain and swelling that began $3$ days ago. He also reports a mild burning sensation when urinating and bilateral eye redness. Four weeks ago, he experienced an episode of severe, watery diarrhea that resolved without medical intervention after several days. His temperature is $37.3^\circ\text{C}$ ($99.1^\circ\text{F}$), blood pressure is $122/78\text{ mmHg}$, and pulse is $84/\text{min}$. Physical examination reveals a warm, swollen, and tender right knee with a palpable effusion. There is exquisite tenderness upon palpation of the left posterior heel at the calcaneal insertion. The conjunctivae are injected bilaterally. Arthrocentesis of the right knee yields synovial fluid with a leukocyte count of $22,000/\text{mm}^3$ and no organisms on Gram stain. Which of the following is the most appropriate next step in management for this patient?