HISTORY: 68-yr old male weightlifter presented with 1 mo. L buttock pain, acute onset of popping mid-lift during a deadlift, with radiating vague pain in L buttock. He continued to exercise through pain and 1 wk later pain progressed to severe deep buttock pain and difficulty ambulating. Denied low back pain, radiating thigh/leg symptoms, numbness/tingling, weakness, or bowel/bladder changes. Past hx positive for melanoma (treated in remote past), social hx negative for smoking, drug/alcohol abuse. He initially went to outside facility (only lumbar x-rays taken) without improvement, came in for another opinion with progressive symptoms. PHYSICAL EXAMINATION: Skin normal without warmth, erythema. Vague tenderness in region of L buttock and SI joint. Full lumbar spine and hip ROM. Negative SLR. Mild nonspecific back discomfort with FABER. Normal lower limb neurovascular exam. DIFFERENTIAL DIAGNOSIS: 1. SI Joint Dysfunction 2. Lumbar Radiculopathy 3. Piriformis Syndrome 4. Lumbar or Sacral Compression Fracture 5. Metastatic Cancer 6. Sacroiliitis TEST AND RESULTS: Outside lumbar x-rays negative for fracture, showed minimal degenerative disc changes. We obtained hip/pelvis radiographs showing significant erosions and destructive changes of L SI joint. An urgent MRI showed fluid-filled and widened SI joint, severe inflammatory sacroiliitis with extensive erosions and bone marrow edema, widespread reactive muscle edema (iliacus showing fluid collection/early abscess at SI joint); SI joint aspirate was obtained. FINAL WORKING DIAGNOSIS: septic sacroiliac arthritis with iliacus abscess and secondary osteomyelitis; aspirate grew staphylococcus aureus (MSSA). TREATMENT AND OUTCOMES: The patient had a unique etiology of osteomyelitis not commonly reported. Orthopedics and ID determined he had pre-existing asymptomatic abscess that ruptured. Under the extreme abdominal pressures of deadlifting, they felt abscess burst leading to local spread and secondary osteomyelitis. This case highlights the rapidity and widespread destruction of infectious arthritis and need for awareness and prompt workup. Patient was treated with 6 wks of oxacillin and responded well. He returned to baseline physical activity at follow up; repeat x-rays showed SI joint sclerosis and patient was counseled on SI joint arthritis.