This patent has a suspected renal stone, the most useful investigation would be an abdominal ultrasound scan (D) specifically of the kidneys, ureters and bladder (KUB). The differential diagnosis should include other causes of abdominal pain, such as acute appendicitis. The most common stone consists of calcium oxalate and up to 80 per cent of renal stones are visible on ultrasound. The KUB US scan is poor at differentiating different stones and can often miss smaller stones. Intravenous urography (IVU) (C) is more useful for detecting obstructions in the renal tract and US scans should always be done before IVU as smaller stones often become nonvisible. IVU scans as it is able to differentialte upto 99 percent of renal stones and it is also useful for exploring other causes of an acute abdomen. However, the radiation risk to the patient makes this investigation much more invasive and is not as readily available as an US scan. An MRI scan (B) has only a few advantages over a CT scan, these include the ability to assess renal aeries and there is no risk form radiation. However, MRI has poor differentiation ability and stones can often appear similar to tumours of blood clots. Where it is paicularly impoant to avoid radiation x-ray (A) is reasonable in detecting renal stones (up to 80 per cent) since the majority consist of calcium which shows up well on radiograph films, however, there is again a radiation risk to the patient which is avoidable in USG scans.