A) Colonoscopy with evaluation of the terminal ileum

The patient's presentation of chronic abdominal pain, nonbloody diarrhea, weight loss, and laboratory findings of anemia, leukocytosis, and elevated ESR is suggestive of an inflammatory bowel disease, such as Crohn disease. Colonoscopy with terminal ileum evaluation for biopsy is the best next step to confirm the diagnosis and assess the extent and severity of the disease.

Answer choice B: CT scan of the abdomen, is incorrect. A CT scan of the abdomen can help identify complications of inflammatory bowel disease but is not the best initial diagnostic test.

Answer choice C: ERCP, is incorrect. ERCP is used to diagnose biliary and pancreatic disorders and is not relevant for this presentation.

Answer choice D: Hydrogen breath test, is incorrect. A hydrogen breath test is used to diagnose lactose intolerance and small intestinal bacterial overgrowth, which are less likely given the systemic inflammatory findings.

Answer choice E: Stool culture and ova and parasite exam, is incorrect. Stool culture and ova and parasite exam can help identify infectious causes of diarrhea but is not the best next step given the chronicity and systemic features of the patient's condition.

Key Learning Point

Inflammatory bowel disease should be suspected in a patient with chronic abdominal pain, non-bloody diarrhea, weight loss, anemia, leukocytosis, and elevated ESR. Colonoscopy with terminal ileum biopsy is the best next step to confirm the diagnosis and assess disease severity.