Co-occurrence of Klinefelter Syndrome and Antisynthetase Polymyositis Complicated by Interstitial Lung Disease

A 15-year-old boy with a medical history of obesity and mild intermittent asthma presented with progressive respiratory distress despite initial treatment of suspected atypical pneumonia. The differential diagnosis was broadened to include an autoimmune cause, and work-up revealed an elevated creatine kinase level and a positive anti–Jo-1 titer, leading to a diagnosis of antisynthetase polymyositis. The patient improved with high-dose steroids and was transitioned to a steroid-sparing regimen before discharge. Fifteen months later, the patient was found to have Klinefelter syndrome. This case highlights the sex bias of autoimmune disease, which is important to consider even in phenotypically male patients.