MRCP2-4571
A 45-year-old man presents with a swollen and painful right knee, along with difficulty in walking for the past three days. Upon examination, the knee appears red, warm, tender, and has limited movement. The knee aspirate shows no organisms on Gram stain, but plenty of leucocytes and weakly positively birefringent crystals on polarised light microscopy. A knee radiograph reveals linear meniscal calcification. Recent blood tests show elevated WBC count, neutrophils, and ESR, along with corrected calcium and phosphate levels outside the normal range. Alkaline phosphatase and urea, electrolytes, and creatinine levels are normal. What is the underlying cause of acute monoarthritis in this patient?