Antibiotics are appropriate for this patient who has an acute inflammatory monoarthritis, possibly infectious arthritis. Acute crystalline attack can also cause fever and an inflammatory monoarthritis, but a negative Gram stain and/or the presence of crystals do not rule out infection. If infection is suspected, even if not yet confirmed on cultures, empiric therapy with antibiotics should be started without delay. The crystals seen in this patient's synovial fluid are extracellular, which is consistent with a diagnosis or history of gout but not diagnostic of an acute gouty attack; only intracellular crystals are diagnostic of an acute crystalline attack. In the absence of positive findings on Gram stain, initial empiric therapy usually includes coverage for gram-positive organisms (including methicillin-resistant Staphylococcus aureus [MRSA], which is increasing in prevalence in many communities) as well as coverage for gram-negative organisms if immunocompromised, at risk for gonococcal infection, or with trauma to the joint. Therapy can be adjusted once results from stains and cultures are available.

Probenecid is sometimes added to allopurinol to control gout when allopurinol alone is insufficient. If the patient were to experience persistent gout, increasing allopurinol to lower the serum urate below 6.0 mg/dL (0.35 mmol/L) would be more appropriate than combination therapy with two agents.

Allopurinol is considered a first-line agent for serum urate reduction in patients with gout. Increasing this patient's allopurinol will not be helpful in treating an acute gouty attack, and it will not treat an infected joint. It may be indicated long term if the patient has persistently elevated serum urate with recurrent attacks of gout.

An intra-articular glucocorticoid injection can be used to treat an acute crystalline attack; however, if an infected joint were suspected, it would not be appropriate because it may worsen infection. It is therefore inappropriate at this time because the cause of this patient's acute inflammatory monoarthritis has not been determined.

Similarly, prednisone should not be started because it may cause worsening of infection. Prednisone is sometimes used short term to treat acute attacks of gout, but if a single large joint is involved, a local injection would avoid the systemic side effects of prednisone therapy.