A 16-year-old boy comes to the office in October due to right knee swelling. The symptoms began yesterday following soccer practice. The patient recalls no specific injury. He has mild associated stiffness. Over the last several months, he has experienced fatigue with occasional flulike illnesses and pain occurring in variable joints. The patient spent the summer at a soccer camp in Maine. He is sexually active with his girlfriend and uses condoms. Vital signs are normal. The patient can bear weight and has a grossly normal gait. Examination of the right knee shows palpable warmth and a large effusion. It is tender to palpation. Range-of-motion testing shows decreased flexion of the right knee compared with the left. All other joints are normal. Plain radiographs reveal no bony deformity. Aspiration yields yellow, translucent fluid with a leukocyte count of 20,000/mm³ (50% neutrophils) and no organisms on Gram stain. Which of the following is the most likely cause of this patient's knee swelling?

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11ec25c8_d2be_5a84_bc81_6de865723f10_MD0003_00 This patient's history of acute monoarticular arthritis following months of migratory arthralgias and a trip to Maine are concerning for Lyme arthritis, the hallmark of late Lyme disease. Lyme disease is caused by Borrelia burgdorferi, a spirochete transmitted by the deer tick (Ixodes scapularis). Disseminated infection occurs via hematogenous spread of infection from the site of the tick bite to various tissues. Maine Early symptoms of Lyme disease (eg, erythema migrans, fever, fatigue) are often overlooked or misdiagnosed, making joint swelling the first apparent manifestation of disease in some patients. After weeks to months of infection, the disease can progress to an inflammatory monoarticular or an asymmetric oligoarticular arthritis, most commonly involving the knee. Patients are typically afebrile and able to bear weight. Effusion is often large and may be associated with pain or stiffness. erythema migrans Synovial fluid analysis in Lyme arthritis shows inflammation with an average leukocyte count of 25,000/mm³; Gram stain and culture are usually negative. Detection of Lyme antibodies in the serum (enzyme-linked immunosorbent assay [ELISA] and Western blot testing) confirms the diagnosis. Oral doxycycline is the first-line treatment. (Choice B) Gonorrhea can cause acute, afebrile, monoarticular arthritis and chronic polyarticular arthralgias. However, gram-negative diplococci are present, and disseminated infection often presents with tenosynovitis and pustular skin lesions. (Choice C) Reactive arthritis presents 1-4 weeks after an enteric or Chlamydia trachomatis infection. Chronic, asymmetric arthritis occurs, typically with urethritis and eye involvement (eg, conjunctivitis, uveitis). (Choice D) Rheumatic fever, a rare complication of streptococcal pharyngitis, is characterized by fever and migratory arthritis. Other manifestations include subcutaneous nodules, erythema marginatum, carditis, and chorea, none of which are seen in this patient. (Choice E) Staphylococcal infection can cause acute monoarthritis. In contrast to this patient's presentation, the presentation of staphylococcal infection includes fever, inability to bear weight, purulent synovial fluid (leukocytosis [>50,000/mm³]), and gram-positive cocci. Educational objective: Lyme arthritis is the hallmark of late Lyme disease due to Borrelia burgdorferi infection. The presentation is most commonly a monoarticular arthritis of the knee that occurs in a weight-bearing, afebrile patient. Synovial fluid analysis shows inflammation, but Gram stain and culture are usually negative. References: Diagnosis and treatment of Lyme arthritis. (http://www.ncbi.nlm.nih.gov/pubmed/25999223) Diagnosis, treatment, and prevention of Lyme disease, human granulocytic anaplasmosis, and babesiosis: a review. (http://www.ncbi.nlm.nih.gov/pubmed/27115378)

A 16-Year-Old Boy Comes to the Office in October Due