Any patient over the age of 40 who presents with unexplained or even if explained Iron deficiency anemia you have to immediately think of CA colon.
Also, any elderly patient with "Acute" constipation or a change in bowel habit you should also think reflexively of CA colon.
Another classic clue is Streptococcus bovis infective endocarditis.
These scenarios are commonly utilized in the make up of USMLE CK questions.
