What is the initial investigation recommended for a patient presenting with Crohn's disease?

For a patient presenting with Crohn's disease, the recommended initial investigation is a CT abdomen. This imaging modality is particularly useful because it provides a detailed view of the intestines and can help identify not only the presence of Crohn's disease but also the complications that may arise, such as strictures, fistulas, or abscesses. The non-invasive nature of CT imaging, along with its ability to capture cross-sectional views, allows for a comprehensive understanding of the disease extent and any associated abnormalities.

Colonoscopy is often used for direct visualization of the colon and may be used in further evaluations, but it is typically not the first-line assessment when presenting symptoms suggestive of Crohn's disease. Similarly, an upper GI series can be helpful in assessing the upper gastrointestinal tract but does not provide as complete a picture of the bowel as a CT. An abdominal ultrasound can be useful in certain scenarios, particularly in pediatrics or to assess complications, but it does not have the same level of sensitivity in diagnosing Crohn's disease compared to a CT scan. Therefore, a CT abdomen is favored as the initial investigation for its broader diagnostic capability in this context.