For a patient with a history of recurrent peri-anal infections, what is the first investigation to order?

The most appropriate initial investigation for a patient with a history of recurrent peri-anal infections is a digital rectal exam. This examination allows the clinician to assess the anal region for any signs of infection, abscess formation, or other abnormalities such as fissures or hemorrhoids that could be contributing to the recurrent infections. It provides immediate and valuable clinical information that can guide further management.

While a CT scan, MRI, and colonoscopy are also important diagnostic tools, they are generally not the first-line investigations in this scenario. A CT scan may be useful for evaluating deeper structures or abscesses, and MRI can provide detailed images of the pelvic anatomy, but these modalities are typically reserved for cases where there is a need for further exploration after a digital exam has raised specific concerns. Colonoscopy is usually indicated in the investigation of bowel issues or suspected inflammatory bowel disease but is not directly relevant to peri-anal infections without further clinical indication.

Thus, starting with a digital rectal exam allows the clinician to gather essential information quickly and make informed decisions regarding additional investigations or treatment options.