A 65-year-old previously well man presents to the clinic with complaints of fatigue of 3-months’ duration. Questioning reveals diffuse weakness and feeling winded when walking uphill or climbing more

A 65-year-old previously well man presents to the clinic with complaints of fatigue of 3-months’ duration. Questioning reveals diffuse weakness and feeling winded when walking uphill or climbing more than one flight of stairs. All of the symptoms have slowly worsened over time. There are no other complaints, and the review of systems is otherwise negative. The patient has no significant medical history, social history, or family history. On physical examination, he appears somewhat pale, with normal vital signs. The physical examination is unremarkable except for his rectal examination, which reveals brown, guaiac-positive stool (suggests the presence of blood in the stool). A blood test reveals anemia.