A 57-year-old man comes to the emergency department due to a 1-day history of pain and swelling in his right knee. The pain is severe, and he has had difficulty bearing weight on the right leg. He previously felt well. Past medical history is notable for hypertension and seasonal allergies. The patient does not use tobacco and drinks 1 or 2 alcoholic beverages a week. On examination, the right knee is red and swollen. He has pain with passive range of motion at the knee. The knee ligaments show no significant laxity. Synovial fluid obtained from the involved joint shows a white blood cell count of 25,000/mm³, with a neutrophilic predominance, and numerous crystals (shown below). These crystals are most likely composed of which of the following substances?

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11ec160b_3a67_4357_9249_a3e776cdcc1e_MD0001_00 Pseudogout, or calcium pyrophosphate deposition disease (CPPD), results from accumulation of calcium pyrophosphate dihydrate crystals in the synovial fluid. Patients present with an acute mono- or oligoarticular arthritis characterized by pain, joint swelling, erythema, and warmth. Pseudogout and gout can be difficult to distinguish clinically. The knee joint is involved in >50% of pseudogout cases, whereas the first metatarsophalangeal joint is more frequently involved in gout. In pseudogout, synovial fluid analysis reveals an elevated white blood cell count with neutrophilic predominance. Identification of rhomboid-shaped calcium pyrophosphate dihydrate crystals is diagnostic. These crystals are positively birefringent under compensated polarized light, meaning that the color pattern is the opposite of that seen in gout. Pseudogout crystals are blue when aligned parallel and yellow when aligned perpendicular to the slow ray of the compensator. (Choice A) Calcific tendonitis results from the deposition of calcium hydroxyapatite crystals in periarticular soft tissues (especially tendons). The rotator cuff tendons are most commonly affected. (Choice B) Calcium oxalate is one of the most common constituents of renal calculi. (Choice D) Soft-tissue deposits of cholesterol and other lipids are called xanthomas. These appear as yellow papules on the knees, elbows, and tendon insertion sites in patients with hyperlipidemia. (Choices E and F) In gout, monosodium urate (the salt of uric acid) is deposited in joints and soft tissues. Urate crystals are needle-shaped and negatively birefringent. Educational objective: Synovial fluid analysis showing rhomboid-shaped calcium pyrophosphate dihydrate crystals is diagnostic of pseudogout. These crystals are positively birefringent under compensated polarized light. The knee joint is involved in >50% of cases.

A 57-Year-Old Man Comes to the Emergency Department Due to a 1-Day