Uncommon Sacroiliac and Presacral Tuberculosis Mimicking Chronic Low Back Pain in a Young Female.

Tuberculosis (TB) of the sacroiliac (SI) joint is an uncommon manifestation of osteoarticular TB, typically presenting with vague, long-standing back pain that resembles mechanical or inflammatory disorders. Because of its deep anatomical location, paucibacillary load, and lack of awareness, diagnosis is often delayed. We describe an 18-year-old female from a TB-endemic area with a 3-year history of persistent low back pain radiating to the left gluteal region. Despite repeated symptomatic treatment, symptoms persisted. Examination showed SI joint tenderness and positive FABER and Gaenslen's tests. Magnetic resonance imaging demonstrated a sacral lesion suggestive of infection. Computed tomography-guided biopsy revealed granulomatous inflammation with caseous necrosis, and Ziehl-Neelsen staining confirmed acid-fast bacilli, establishing SI and presacral TB. She was started on first-line antitubercular therapy (ATT) and improved with conservative management. This report emphasizes the diagnostic difficulty of SI joint TB, particularly in young patients with unexplained back pain in endemic settings. While imaging is useful, tissue sampling remains the gold standard. Although classification systems often recommend surgical intervention for advanced disease, our patient responded well to medical treatment alone, highlighting the role of conservative therapy. Early biopsy-based diagnosis and timely ATT are crucial to avoid complications.