Question

An otherwise healthy $26$-year-old male is evaluated in the outpatient clinic due to a $4$-day history of a swollen, painful, and warm right knee. He also reports recent onset of left heel pain that worsens with walking. For the past week, he has experienced bilateral eye redness and a "gritty" sensation. Three weeks prior to this visit, the patient had a self-limited episode of fever, abdominal cramps, and watery diarrhea that resolved after a few days. His current temperature is $37.4^\circ\text{C}$ ($99.3^\circ\text{F}$), blood pressure is $122/78$ mmHg, and pulse is $86$/min. Physical examination shows a large effusion in the right knee with restricted range of motion secondary to pain. There is marked tenderness at the insertion of the left Achilles tendon. Bilateral conjunctival injection is noted without purulent discharge. Arthrocentesis of the right knee yields $40$ mL of cloudy yellow fluid. Synovial fluid analysis reveals a leukocyte count of $35,000/\text{mm}^3$ with $80\%$ neutrophils. Gram stain shows no organisms, and polarized light microscopy demonstrates no crystals. Which of the following is the most appropriate next step in the management of this patient?