Figure 5a. Renal abscess in a 26-year-old man with acquired immunodeficiency syndrome who presented with right flank pain. (a) Initial unenhanced helical CT scan shows no obstructing stone but demonstrates a hypoattenuating mass in the right upper pole. (b) Delayed contrast-enhanced CT scan helps confirm the presence of a cystic lesion with adjacent renal parenchymal edema (arrows). Staphylococcus aureus was seen in cultures of the aspirated specimen.