Acute Bacterial Sacroiliitis With Associated Iliacus Abscess in a Healthy Young Woman: A Case Report.

Background: Acute bacterial sacroiliitis is a rare yet potentially debilitating infection of the sacroiliac (SI) joint. While often associated with immunocompromised states, intravenous drug use, or trauma, this condition can also manifest in, otherwise, healthy individuals. Its nonspecific clinical presentation frequently mimics common musculoskeletal disorders, such as mechanical back pain or inflammatory arthropathies, leading to underdiagnosis and treatment delays. Objective: This case report presents a rare instance of acute bacterial sacroiliitis in a previously healthy, immunocompetent adult female with no predisposing conditions. The report emphasizes the diagnostic challenges, the role of imaging, and the necessity for early intervention. Case Presentation: A 40-year-old woman with no significant medical history presented with a 21-day history of progressively worsening pain in the left SI joint, radiating to the left lower limb, accompanied by fever, anorexia, and chills. Initial treatment with analgesics and nonsteroidal anti-inflammatory drugs proved ineffective, and laboratory tests revealed significantly elevated inflammatory markers. Magnetic resonance imaging (MRI) demonstrated joint effusion, a multiloculated abscess within the left iliacus muscle, and adjacent myositis. Ultrasound-guided arthrocentesis confirmed Conclusion: This case highlights the importance of maintaining a high index of suspicion for bacterial sacroiliitis in immunocompetent adults presenting with persistent pelvic pain. Prompt diagnosis through advanced imaging and targeted therapy is essential to prevent complications and ensure favorable clinical outcomes.