With multi-spatial rim enhancing collections with intramuscular extension correlating with raised infective markers and newly diagnosed diabetes mellitus, an abscess is the top differential diagnosis. It is unlikely to be a retroperitoneal malignancy in view of the lack of aggressive features such as bony erosion and enhancing solid components.

Incidental findings of filling defects in the right lower lobe pulmonary artery and subsegmental branches of the posterobasal segment left lower lobe pulmonary artery are in keeping with acute pulmonary embolism (PE).

The abscess was drained with an ultrasound-guided pigtail drainage catheter. The acute PE was treated with anticoagulants.

Klebsiella pneumoniae was isolated from the drained pus, while Candida albicans was isolated from the urine. K. pneumoniae is the second most frequent bacterial organism in urinary tract infection, after Escherichia coli, and the close relation between the chronically obstructed left kidney and the large left retroperitoneal/gluteal abscesses, the etiology of the abscess is likely arising from a left infected obstructed kidney.

This case depicts the importance of having a systematic review of all areas in CT scan, though the main indication for this CT scan is for assessing the extension of the retroperitoneal/subcutaneous collection, the incidental finding of acute pulmonary embolism is very crucial for the patient's management. In view of the patient's risk factor of immobility due to sciatica for the past 1 week, this has increased the chances of pulmonary embolism, the assessment of the imaged pulmonary arteries should be included in routine assessment.