This is meant to be Charlie's first colonoscopy, but in "The Devil's Lube" (s6e11), he phones his doctor for his test results. During the call, he states, "I peed in a cup, you took my blood and slipped a camera up the heart of darkness."
At least the second time a colonoscopy has been used as a source for humor on the show. The first was Alan's impending colonoscopy in season 2, which sparks a trip to Las Vegas as a distraction. In an article for Vanity Fair, Jon Cryer is quoted as having said that much of writer Chuck Lorre's comic perspective can be traced to a colonoscopy he received at the hands of medical students.
Episode title spoken by Alan, to Charlie.