Unraveling Atypical Osteomyelitis: A Rare Group B Streptococcus Infection in an Adolescent With Diagnostic Dilemma.

Group B Streptococcus (GBS) is a well-recognized pathogen in neonates and older adults but is rare in adolescents, particularly as a cause of musculoskeletal infection. We describe the case of a previously healthy 16-year-old female patient with GBS sacroiliac septic arthritis and osteomyelitis presenting with progressive hip and back pain complicated by acute urinary incontinence. Initial evaluation was confounded by nonspecific symptoms, under-recognized inflammatory markers, and incidental spinal imaging findings, resulting in diagnostic delay. Blood cultures ultimately identified GBS, and targeted antimicrobial therapy led to clinical improvement. Subsequent imaging and laboratory findings raised concern for chronic osteomyelitis, prompting prolonged therapy with full recovery. This case highlights an unusual presentation of adolescent GBS osteomyelitis involving the sacroiliac joint and acute urinary incontinence, an association not previously reported. It underscores the importance of maintaining a broad differential, interpreting laboratory abnormalities in context, avoiding diagnostic anchoring, and obtaining site-directed imaging in adolescents with persistent musculoskeletal pain and systemic inflammation.