A 28-year-old female presents with a tender, swollen, and hot left ankle and synovial fluid showing 55,000 WBCs. What is the most likely diagnosis?

In the context of a 28-year-old female with a tender, swollen, and hot left ankle, along with synovial fluid analysis revealing a high white blood cell count of 55,000, the most likely diagnosis is septic arthritis.

Septic arthritis is characterized by the presence of infection in the joint, which typically results in significant inflammation. The high white blood cell count in the synovial fluid, particularly with a predominance of neutrophils, is a strong indicator of infection. A count of 55,000 WBCs is notably elevated and suggests a severe inflammatory response commonly associated with septic conditions.

The other potential diagnoses can be ruled out based on the clinical context and laboratory findings. For example, rheumatoid arthritis usually presents with a lower WBC count and would not typically cause such an acute, localized, and severe inflammatory response leading to significant swelling and heat in one joint. Gouty arthritis might also cause joint swelling and pain but typically does not produce such a high leukocyte count nor is it associated with fever and systemic signs of infection as often as septic arthritis. Osteoarthritis tends to be a chronic, degenerative joint disease that usually does not present acutely with such pronounced symptoms or this level of inflammatory activity.