A 33-year-old man comes to the emergency department due to a 2-day history of left knee pain and swelling. The patient has not had similar symptoms before and reports no preceding trauma, pain in other joints, skin rash, fever, or chills. Medical history is notable only for seasonal allergies, for which he takes loratadine as needed. The patient does not use tobacco, alcohol, or illicit drugs. Temperature is 36.7 C (98 F), blood pressure is 110/56 mm Hg, pulse is 78/min, and respirations are 14/min. BMI is 22 kg/m². Physical examination shows a moderate-sized left knee effusion. The knee is slightly warm but not erythematous. There is no locking of the joint or ligamentous laxity. Laboratory results are as follows: X-ray of the left knee reveals linear calcifications of the menisci and articular cartilage with a moderate effusion but no fracture. Synovial fluid analysis shows leukocyte count of 15,400/mm³ with 80% neutrophils. Gram stain shows no organisms. Crystal analysis is pending. Which of the following additional tests is most appropriate in evaluation of this patient's symptoms?

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11ec193a_96b3_67c4_853d_db4c4d2b304b_MD0002_00 This patient has an acute inflammatory arthritis associated with chondrocalcinosis. Although microscopy results from the synovial fluid are not yet available, they are likely to show rhomboid-shaped calcium pyrophosphate dihydrate crystals consistent with pseudogout. Pseudogout is most common in the elderly. Occurrence in a young or middle-aged man suggests possible secondary causes, such as hemochromatosis. Hemochromatosis causes reduced clearance of calcium pyrophosphate crystals from the synovial fluid. Therefore, patients can develop any aspect of calcium pyrophosphate deposition disease, ranging from the chondrocalcinosis and the classic polyarthropathy (hook-like osteophytes most prominent in the 2nd and 3rd metacarpophalangeal joints) to acute pseudogout. Given this patient's relatively young age, he should be screened for hemochromatosis with serum iron studies (ie, serum iron, total iron-binding capacity, ferritin). (Choices A and B) Rheumatoid arthritis typically causes a chronic, slowly progressive arthritis predominantly affecting the metacarpophalangeal joints and wrists. Systemic lupus erythematosus can also cause chronic polyarticular arthritis, often with additional systemic symptoms. This patient's acute monoarticular arthritis of the knee is unlikely to represent autoimmune disease, and serologic studies are not indicated. (Choice C) HLA-B27 is associated with ankylosing spondylitis (eg, back pain and stiffness, sacroiliitis, enthesitis), reactive arthritis (eg, oligoarthritis, urethritis, conjunctivitis), psoriatic arthritis (eg, progressive oligo- or polyarthritis, spondyloarthropathy), and juvenile idiopathic (rheumatoid) arthritis (eg, episodic joint pain, swelling, stiffness). All these disorders cause subacute to chronic symptoms rather than acute inflammatory monoarthritis. (Choice E) Although gout can present with acute monoarthritis of the knee, this patient has no significant risk factors for gout, and the chondrocalcinosis makes pseudogout more likely. In addition, serum uric acid levels should not be drawn during an acute attack as they may be misleadingly normal. Educational objective: Pseudogout is characterized by acute inflammatory arthritis associated with chondrocalcinosis. The knee is the most commonly affected joint. Pseudogout is most common in the elderly; onset in a young patient should prompt testing for secondary causes, such as hemochromatosis. __________ References: An update on the epidemiology of calcium pyrophosphate dihydrate crystal deposition disease. (http://www.ncbi.nlm.nih.gov/pubmed/19398486) HFE-related hemochromatosis: an update for the rheumatologist. (http://www.ncbi.nlm.nih.gov/pubmed/24264720)

A 33-Year-Old Man Comes to the Emergency Department Due to a 2-Day