LAUSR

A 7-year-old boy with recent left elbow septic arthritis and osteomyelitis presented with new left knee swelling. The patient was recovering from recurrence of traumainduced left elbow septic arthritis and osteomyelitis, which had resulted in 2 admissions within the preceding 2 months. Approximately 1 month into his antibiotic treatment for these conditions, he had developed Clostridioides difficile (C difficile) enterocolitis and was treated with metronidazole. Presenting symptoms began with left knee pain 3 days prior to presentation, with subsequent redness, swelling, pain, and refusal to ambulate, prompting a visit to the Emergency Department. At this time, he was still completing a 6-week course of cephalexin 3 times daily for prior diagnosis. He had experienced mild back pain for approximately 2 weeks prior to admission but had no other significant musculoskeletal complaints. He had no rashes, no visual symptoms, no urogenital symptoms, and no current complaints of diarrhea, after his C difficile diagnosis and treatment 3 weeks prior. At the Emergency Department, he was afebrile (T 36.7°C), had tachycardia to 125 bpm, with normal respiratory rate (26 bpm), blood pressure (102/77 mm Hg), and oxygen saturating 98% on room air. Physical examination was notable for erythema of the left knee, with warmth, swelling, and tenderness to palpation, as well as a palpable effusion. Patient preferred to keep the left leg flexed at the knee and reported pain with full extension. Neurovascular examination was intact. He had negative examination at all other joints, except his left elbow which showed only mild swelling, but no tenderness or limited range of motion, felt to be secondary to improving previous infection. Diagnostic testing showed evidence of inflammation, with white blood cell (WBC) count of 14 000/uL, elevated platelets to 627 000/uL, erythrocyte sedimentation rate to 60 mm/h, C-reactive protein (CRP) reaching a peak of 15.9 mg/dL. Comprehensive metabolic panel was within normal limits. He was SARS-COv-2 PCR (polymerase chain reaction) negative. The x-ray of the left knee showed suprapatellar joint effusion but no acute fracture and no radiographic evidence for acute osteomyelitis. Magnetic resonance imaging (MRI) of the left knee identified joint effusion with abnormal synovial enhancement but no concern for osteomyelitis, no internal debris or loculations, nor an abnormal marrow signal (Figure 1). He underwent an incision and debridement with Orthopedic Surgery with synovial fluid cell count of WBC 13 000 cells/mm3 (83% segs) and red blood cell count 3000 cells/mm3. Bacterial and fungal joint fluid cultures were negative. His bacterial blood cultures were also negative. Final Diagnosis: Reactive arthritis (ReA) secondary to C difficile enterocolitis.