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The first steps would be to first make a firm diagnosis. It is difficult to know what should be done regardnig diet or treatment until a proper diagnosis is made.
The primary test that would detect Crohn's disease and other forms of colitis would be a colonoscopy. A colonoscopy would also be able to assess the current status of diverticula. Doctors will usually wait until an acute diverticulitis has resolved before performing a colonoscopy, but it will ultimately be the appropriate test to look for Crohn's disease or colitis.
The preferred test to diagnose lactose intolerance is a breath test after ingestion of lactose. Another option would be to follow a lactose-free diet to see if symptoms improve.
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