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A 68-year-old man with poorly controlled type 2 diabetes and a 10-year history of rheumatoid arthritis presents to the emergency department with a 24-hour history of an acutely hot, swollen, exquisitely tender right knee. He is unable to bear weight and holds the knee in slight flexion. On examination he is febrile (38.9 degrees C), tachycardic, and the right knee is warm, diffusely tender with a large effusion and severe pain on any passive movement. His inflammatory markers are raised (CRP 220 mg/L, WBC 17.8).
Questions
a) What is the most likely diagnosis and the single most urgent investigation? (2 marks)
Most likely septic arthritis (must be assumed in any acute hot, swollen, tender monoarthritis, especially in a high-risk patient with RA and diabetes). The single most urgent investigation is joint aspiration — sent for cell count and differential, Gram stain, culture and crystal microscopy — performed before antibiotics are given.
b) List four further investigations you would arrange. (2 marks)
- Blood cultures (two sets, before antibiotics)
- FBC, CRP, ESR, U&E, LFT, glucose, lactate, coagulation
- Synovial fluid Gram stain, culture and crystal microscopy (from aspiration)
- X-ray of the knee (baseline; exclude osteomyelitis/foreign body) ± ultrasound/MRI if the diagnosis is unclear or osteomyelitis suspected
c) Outline the acute management. (4 marks)
- Resuscitate if septic — oxygen, IV fluids, monitor; sepsis bundle if shocked.
- Aspirate the joint before antibiotics (diagnostic ± therapeutic).
- Urgent orthopaedic referral for surgical washout/drainage (arthroscopy or arthrotomy) — the mainstay.
- Empirical IV antibiotics after aspiration — vancomycin + Gram-negative cover (e.g. ceftriaxone), then narrow to culture sensitivities; total 4 to 6 weeks (about 2 IV then oral).
- Analgesia, splint in position of comfort, then early mobilisation and physiotherapy once settled; treat source and optimise diabetes.
d) Name two complications and one factor that worsens prognosis. (2 marks)
- Complications: permanent articular cartilage destruction / secondary osteoarthritis, sepsis and septic shock, osteomyelitis, joint stiffness/contracture, amputation, death (mortality 7 to 15%).
- Prognosis worsened by delay in treatment beyond 24 to 48 hours (other factors: RA, prosthetic joint, polyarticular disease, immunosuppression, elderly, S. aureus or Gram-negative organisms).