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Q1: Presentation & diagnosis (2 min)
A 65-year-old presents with an acutely hot, swollen, tender knee and fever. Take us through your approach. Why is septic arthritis an emergency, and what is the single most important investigation? Cover: the "septic until proven otherwise" rule, urgent joint aspiration (before antibiotics), synovial fluid interpretation (WBC over 50,000, neutrophil-predominant, positive Gram stain/culture, low glucose), and why crystals do not exclude infection.
Q2: Organisms & risk factors (2 min)
Which organisms cause septic arthritis and in whom? Cover: Staphylococcus aureus commonest (incl MRSA); Neisseria gonorrhoeae in young sexually active adults (migratory polyarthralgia, tenosynovitis, pustular rash — diagnose with NAAT); Gram-negatives and Pseudomonas in IVDU (unusual joints: sternoclavicular, sacroiliac, spine); coagulase-negative staph in prosthetic joints. Risk factors: prosthetic joint, rheumatoid arthritis, diabetes, immunosuppression, IV drug use, age over 80, skin infection, recent injection/surgery.
Q3: Management (3 min)
How do you manage confirmed septic arthritis? Cover: resuscitation if septic; aspirate before antibiotics; urgent surgical washout/drainage (arthroscopy/arthrotomy — mainstay); empirical IV vancomycin + Gram-negative cover, narrowed to culture, total 4 to 6 weeks (about 2 IV then oral); analgesia, rest then early mobilisation; gonococcal regimen (ceftriaxone + azithromycin/doxycycline); prosthetic joint specialist pathway (debridement, antibiotic spacer, two-stage revision, long suppressive antibiotics).
Q4: Complications & prognosis (2 min)
What are the complications and what determines outcome? Cover: permanent cartilage/joint destruction, secondary osteoarthritis, avascular necrosis, sepsis/septic shock, osteomyelitis, disability/amputation, death (mortality 7 to 15%, up to half retain joint damage). Outcome hinges on time to treatment — delay beyond 24 to 48 hours markedly worsens outcome; worse in RA, prosthetic joints, polyarticular disease, immunosuppressed and elderly.
Q5: Differentiating crystal arthropathy (1 min)
How do you distinguish septic arthritis from gout and pseudogout on synovial fluid? Why does this distinction matter? Cover: gout = negatively birefringent needle crystals; CPPD = weakly positively birefringent rhomboid crystals; septic = purulent, WBC over 50,000 neutrophilic, positive Gram stain/culture. Emphasise: crystals never exclude sepsis — always culture, and treat for infection if any doubt.