What is the presumed diagnosis for a patient with acute monoarthropathy and a synovial fluid WBC count > 50,000/mm3?

The presumed diagnosis for a patient with acute monoarthropathy and a synovial fluid white blood cell (WBC) count greater than 50,000/mm³ is septic arthritis. This condition is characterized by an infection in the joint space, which leads to a significant inflammatory response. The markedly elevated WBC count in the synovial fluid is a strong indicator of infection, as it suggests a high concentration of immune cells working to combat the pathogens present.

In the context of acute monoarthropathy, the presence of such a high WBC count points towards septic arthritis as the underlying cause. The infection can be due to various pathogens, including bacteria, and the presence of infection often necessitates urgent intervention, such as aspiration of the joint and antibiotic therapy.

Other conditions like rheumatoid arthritis and psoriatic arthritis typically present with lower synovial fluid WBC counts, as these disorders are primarily autoimmune rather than infectious. Osteoarthritis, on the other hand, is also less likely to result in such elevated WBC counts and usually presents as degenerative joint disease without the acute inflammatory response seen in infections. Thus, given the clinical picture and lab findings, septic arthritis is the most appropriate diagnosis.