Moraxella catarrhalis Septic Arthritis Unveils Undiagnosed Systemic Lupus Erythematous in a Pediatric Patient.

Septic arthritis is uncommon in pediatric patients, who are less likely to have major risk factors such as underlying joint disease or prosthetic joints. It only rarely affects the elbow and is usually caused by Gram-positive cocci, with Staphylococcus aureus being the most common bacterial organism. We present the case of a 15-year-old previously healthy female who experienced new-onset monoarticular nontraumatic elbow pain and was found to have a synovial effusion growing from Moraxella catarrhalis. The atypical clinical presentation, coupled with the growth of an unusual organism, raised concern for an underlying immunocompromising or inflammatory joint disorder. Further laboratory workup ultimately revealed a diagnosis of systemic lupus erythematosus (SLE), which more commonly presents with arthralgias that are polyarticular, symmetric, and migratory. This case report should encourage clinicians to maintain a high degree of suspicion for underlying joint disease when septic arthritis presents atypically.