Question

A $28$-year-old man comes to the clinic with a $5$-day history of right knee pain and swelling, accompanied by bilateral eye redness and a burning sensation during urination. He has a medical history of major depressive disorder, for which he takes daily fluoxetine. Four weeks ago, he traveled to Southeast Asia and experienced a self-limited episode of severe watery diarrhea. His temperature is $37.6^\circ\text{C}$ ($99.7^\circ\text{F}$), blood pressure is $122/78\text{ mm Hg}$, and pulse is $88/\text{min}$. Physical examination shows bilateral conjunctival injection with no purulent discharge. The right knee is markedly swollen, warm, and tender to palpation. There is also tenderness over the posterior aspect of the left heel. Arthrocentesis of the right knee yields yellow, translucent synovial fluid with a leukocyte count of $15,000/\text{mm}^3$ ($50\%$ neutrophils). Gram stain and cultures of the synovial fluid are negative. Polarized light microscopy reveals no crystals.

Laboratory studies show:
Leukocyte count: $11,500/\text{mm}^3$
Hemoglobin: $13.8\text{ g/dL}$
Thyroid-stimulating hormone (TSH): $0.3\text{ \mu U/mL}$ (N: $0.5$–$5.0\text{ \mu U/mL}$)
Free T4: $0.7\text{ ng/dL}$ (N: $0.9$–$1.7\text{ ng/dL}$)

Which of the following is the most appropriate next step in management?