In PUO, an occult abscess is a common infectious cause, and the work-up includes CT scan of the chest and abdomen. When an intra-abdominal source is suspected, CT abdomen and pelvis is the investigation that can detect abscesses or other collections throughout the abdomen. Plain abdominal X-ray is insensitive, liver ultrasound only looks at one organ, and a gastric emptying study is irrelevant.