MRI features most consistent with iliacus muscle abscess.

Ultrasound-guided percutaneous drainage was performed with aspiration of 80 cc of frank pus and sent for microbiological analysis.

Psoas muscle abscess may be classified as primary or secondary depending on the presence or absence of underlying disease:

•  primary psoas muscle abscess: can occur in patients with diabetes mellitus, intravenous drug use, AIDS, renal failure or immunosuppression

• secondary psoas muscle abscess: from appendicitis, diverticulitis, Crohn's disease, perforated colon carcinoma, or neighboring spondylodiscitis