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Stem
A 76-year-old woman is admitted to the orthopaedic ward five days after a hemicolectomy. She now has acute, severe pain and swelling of the right knee that began overnight. On examination the knee is hot, erythematous, swollen and exquisitely tender with a large effusion and restricted movement. She is afebrile. Bloods show CRP 110 mg/L, WBC 13.2 x 10^9/L (neutrophilia). She has stage 3 chronic kidney disease (eGFR 45).
Questions
a) What is the most likely diagnosis and what single bedside investigation is diagnostic? (2 marks) Acute calcium pyrophosphate (CPP) crystal arthritis ("pseudogout"), precipitated by recent surgery in an older patient. The diagnostic investigation is joint aspiration with compensated polarised-light microscopy of synovial fluid — showing rhomboid, weakly positively birefringent crystals (blue when parallel to the compensator axis), with an inflammatory fluid (neutrophils) and a NEGATIVE Gram stain and culture.
b) List three differential diagnoses you must actively consider, and why aspiration matters. (3 marks)
- Septic arthritis — the must-not-miss; can coexist with CPPD; excluded only by Gram stain/culture of aspirated fluid.
- Gout — needle-shaped, strongly negatively birefringent urate crystals; typically first MTP.
- Reactive arthritis / haemarthrosis / OA flare — aspirate distinguishes crystal type and excludes infection. Aspiration is pivotal because septic arthritis and pseudogout are clinically indistinguishable and may coexist.
c) Outline your management of this acute episode, justifying your drug choice given her renal function. (3 marks)
- Confirm sepsis is excluded (Gram stain/culture) before intra-articular steroid.
- Intra-articular corticosteroid injection of the knee is the preferred option here — effective for a single large joint and avoids NSAID nephrotoxicity/GI risk in an elderly patient with CKD.
- Alternatively a short course of oral glucocorticoids (e.g. prednisolone 30 mg tapering over 1–2 weeks).
- Colchicine could be used at a reduced renal dose if given early, but is less attractive in CKD.
- NSAIDs should be avoided given her eGFR 45 and age.
- Rest, ice, analgesia; treat any precipitant.
d) What metabolic screen would you arrange and why? Outline long-term management. (2 marks) Screen for metabolic associations (especially relevant if the patient were under 55, but reasonable here): serum calcium and PTH (hyperparathyroidism), iron studies/ferritin (haemochromatosis), magnesium (hypomagnesaemia), TSH (hypothyroidism) and phosphate. Long-term: treat any cause found (e.g. venesection for haemochromatosis); low-dose colchicine prophylaxis for recurrent or peri-operative flares; physiotherapy and OA management. There is no drug that dissolves CPP crystals.