A 9-year-old boy is brought to the office by his parents due to knee pain. His mother states, "He started having occasional left knee pain with mild swelling 2 weeks ago, but the pain and swelling have worsened over the last 5 days." The patient plays basketball but has had no recent trauma. Last week, he had a runny nose and sore throat that resolved on their own. The patient has had no fever, headaches, or chest pain. His mother notes that he has been sleeping "a little more than usual" over the last month. The patient recalls a tick bite 3 months ago during a camping trip in Pennsylvania. He does not remember developing a rash at that time. The patient has congenital hearing loss for which he wears a hearing aid. He takes no daily medications and has no known allergies. Temperature is 36.7 C (98 F) and blood pressure is 100/70 mm Hg. Physical examination shows a well-appearing child with a moderately swollen, warm, and mildly tender left knee. There is no erythema. Minimal pain is elicited with full extension of the knee. The patient is able to stand and bear weight. Sensation is intact. Capillary refill in the bilateral lower extremities is brisk. There is a small bruise on his right knee but no rashes or petechiae. Laboratory results are as follows:
A left knee arthrocentesis is performed, and synovial fluid analysis reveals a white blood cell count of 24,000/mm³ with 85% neutrophils.
Which of the following tests is most likely to diagnose this patient's condition?

Question

A 9-year-old boy is brought to the office by his parents due to knee pain.  His mother states, "He started having occasional left knee pain with mild swelling 2 weeks ago, but the pain and swelling have worsened over the last 5 days."  The patient plays basketball but has had no recent trauma.  Last week, he had a runny nose and sore throat that resolved on their own.  The patient has had no fever, headaches, or chest pain.  His mother notes that he has been sleeping "a little more than usual" over the last month.  The patient recalls a tick bite 3 months ago during a camping trip in Pennsylvania.  He does not remember developing a rash at that time.  The patient has congenital hearing loss for which he wears a hearing aid.  He takes no daily medications and has no known allergies.  Temperature is 36.7 C (98 F) and blood pressure is 100/70 mm Hg.  Physical examination shows a well-appearing child with a moderately swollen, warm, and mildly tender left knee.  There is no erythema.  Minimal pain is elicited with full extension of the knee.  The patient is able to stand and bear weight.  Sensation is intact.  Capillary refill in the bilateral lower extremities is brisk.  There is a small bruise on his right knee but no rashes or petechiae.  Laboratory results are as follows:
 A left knee arthrocentesis is performed, and synovial fluid analysis reveals a white blood cell count of 24,000/mm³ with 85% neutrophils.
Which of the following tests is most likely to diagnose this patient's condition?

Quizplus · Accepted Answer

Explanation: 11ec2b69_bfaa_e054_8f63_0bf4956eedf7_MD0005_00 This patient had a tick bite in Pennsylvania 3 months ago and now has left knee swelling and synovial fluid evidence of inflammation, raising strong suspicion for Lyme arthritis, the most common late complication of Lyme disease. Lyme disease is a spirochetal infection caused by Borrelia burgdorferi. It is transmitted by the deer tick Ixodes scapularis during blood feeding. Most cases occur in rural parts of the upper midwestern and northeastern United States, where the tick is endemic. Initial transmission is often (~80%) marked by a spreading, annular rash (erythema migrans) and systemic symptoms; however, asymptomatic or unrecognized cases often go untreated, leading to persistent infection. upper midwestern and northeastern United States Approximately 60% of patients with untreated Lyme disease develop Lyme arthritis months or years later. A waxing/waning monoarticular arthritis in the knee is the most common manifestations but sometimes a few large joints are affected at once. In contrast to patients with forms of infectious arthritis, those with Lyme arthritis tend to be afebrile and well-appearing and can bear weight on the affected joint. However, arthrocentesis is generally performed because examination findings (eg, tenderness, swelling, decreased range of motion) can overlap with potentially destructive septic arthritis. In Lyme arthritis, the synovial fluid white blood cell (WBC) count is typically 20,000-60,000/mm³. The diagnosis is made by serum enzyme-linked immunosorbent assay and Western blot. (Choice A) Most bacterial joint infections cause fever, severe pain, and synovial fluid leukocyte count >50,000/mm³. Diagnosis is made with bacterial joint fluid culture. B burgdorferi does not grow on standard bacterial joint culture (it requires specialized culture media that is not available in most clinical laboratories). (Choice B) Plain radiographs are not generally diagnostic in children with an inflammatory joint effusion. In a patient with traumatic injury or no effusion, x-rays are sometimes ordered to evaluate for bony pathology (eg, fracture, neoplasm, degenerative disease). (Choice C) Serum anti-citrullinated peptide antibodies may be positive in aggressive cases of polyarticular juvenile idiopathic arthritis, an inflammatory arthritis involving multiple symmetrical joints. This patients monoarticular joint effusion in a the knee would be uncommon. (Choice D) Serum antistreptolysin titers are elevated with acute rheumatic fever, which typically presents with migratory polyarthritis rather than arthritis involving a single joint. In addition, patients are often febrile with an erythrocyte sedimentation rate ≥60 mm/h. Educational objective: Lyme arthritis is the most common late complication of Lyme disease. Suspicion should be raised when a patient in a Lyme-endemic area develops monoarticular or oligoarticular joint pain and swelling. Fever and other systemic symptoms are rare; joint aspiration generally reveals a leukocyte count of 20,000-50,000/mm³. Positive Lyme serology is diagnostic. References: Distinguishing Lyme from septic knee monoarthritis in Lyme disease-endemic areas. (http://www.ncbi.nlm.nih.gov/pubmed/23420916) Predictive factors for differentiating between septic arthritis and Lyme disease of the knee in children. (http://www.ncbi.nlm.nih.gov/pubmed/27147684) Diagnosis and treatment of Lyme arthritis. (http://www.ncbi.nlm.nih.gov/pubmed/25999223)

A 9-Year-Old Boy Is Brought to the Office by His