Sacroilitis: an unusual cause for a common presentation.

Inflammatory arthritis and sacroilitis are common presentations to rheumatology clinics. Owing to the physiological changes of pregnancy, the first presentation can be post partum with back pain and an accompanying oligoarthritis or polyarthritis. We present a woman with lower back pain who demonstrated clinical and radiological features consistent with sacroilitis and an inflammatory arthritis but was found to have an unusual presentation of another common cause of arthritis.