A 68-year-old man presents with sudden severe pain in his left knee. Synovial fluid analysis shows positively birefringent rhomboid crystals under polarised light microscopy. His serum calcium is 2.8 mmol/L. The most likely underlying cause to investigate is:
- A Gout (urate crystal deposition)
- B Haemochromatosis
- C Hyperparathyroidism ✓
- D Hypothyroidism
Correct answer: C. Hyperparathyroidism
Explanation
Positively birefringent rhomboid crystals are calcium pyrophosphate dihydrate (CPPD) — the crystals of pseudogout. CPPD deposition disease is associated with metabolic conditions including hyperparathyroidism (the most common metabolic association when hypercalcaemia is present), haemochromatosis, hypomagnesaemia, and hypophosphatasia. The elevated calcium here points to hyperparathyroidism as the underlying cause to exclude first.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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