Differential Diagnosis for Atraumatic Right Knee Pain

The patient's presentation of atraumatic right knee pain, accompanied by systemic symptoms such as low-grade fevers, night sweats, and localized signs of inflammation (swelling, heat, erythema), suggests an infectious or inflammatory process. Given the patient's history of a bioprosthetic aortic valve replacement, the risk of infectious complications, including endocarditis and septic arthritis, must be considered.

• Single Most Likely Diagnosis

  • Septic Arthritis: This is the most likely diagnosis given the acute onset of severe monoarticular pain, swelling, heat, and erythema, along with systemic symptoms such as fever and night sweats. The patient's inability to tolerate palpation or manipulation of the knee further supports this diagnosis, as septic arthritis typically presents with significant pain and decreased range of motion.

• Other Likely Diagnoses

  • Crystal Arthropathy (Gout or Pseudogout): Although less likely given the systemic symptoms, crystal arthropathies can cause acute, severe monoarticular pain and inflammation. The absence of a history of similar episodes or known crystal disease makes this less likely but still a consideration.
  • Inflammatory Arthritis: Conditions like rheumatoid arthritis or seronegative spondyloarthropathies could present with knee pain and swelling, but the acute onset and systemic symptoms in this case make septic arthritis more likely.

• Do Not Miss Diagnoses

  • Prosthetic Joint Infection: Although the patient's symptoms are localized to the native knee and not the site of any prosthetic joint (given the bioprosthetic aortic valve), any infection in a patient with a prosthetic device warrants consideration of device infection.
  • Endocarditis with Embolic Phenomenon: Given the patient's history of bioprosthetic aortic valve replacement, endocarditis is a critical diagnosis not to miss. Emboli from infected heart valves can cause peripheral symptoms, including joint pain, although this would be less common.
  • Osteomyelitis or Soft Tissue Infection: While less likely given the joint-specific symptoms, osteomyelitis or a soft tissue infection could present with similar signs of inflammation and systemic illness.

• Rare Diagnoses

  • Lyme Arthritis: This could be considered in endemic areas and with a history of tick exposure, but it typically presents with a more subacute onset and may not have the same level of systemic symptoms.
  • Hemarthrosis: Although rare without trauma, especially in the absence of coagulopathy or anticoagulant use, it could be considered if there's a suspicion of underlying bleeding disorder or use of anticoagulants not mentioned in the history.
  • Neoplastic or Paraneoplastic Processes: These are rare causes of monoarticular arthritis and would typically have a more insidious onset, possibly accompanied by systemic symptoms such as weight loss or fatigue.