Apreviously healthy 47-year-old man comes to the physician because of a 6.8-kg (15-lb) weight loss over the past 6 months. he spent 2 weeks in mexico 3 months ago. since returning, he has noticed that his stools have changed in size and consistency. he has not had fever, night sweats, or change in appetite. he takes no medications. he has smoked one pack of cigarettes daily for 20 years. he appears healthy and well nourished. his temperature is 37°c (98.6°f), pulse is 105/min, respirations are 16/min, and blood pressure is 130/78 mm hg. examination shows pale conjunctivae. the abdomen is soft with no organomegaly. rectal examination shows a normal prostate with no masses. test of the stool for occult blood is positive. laboratory studies show: hemoglobin 11 g/dlmean corpuscular volume 72 μm3platelet count 300,000/mm3red cell distribution width 16% (n=13%-15%)which of the following is the most appropriate next step in diagnosis? (a) second complete blood count in 3 months(b) ct scan of the abdomen(c) colonoscopy(d) (e) sigmoidoscopy