This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: G.I.T, Subject name: Surgery
Tumor marker for Ca colon for follow up 
Select and explain. 
A. CEA, B. CA 125, C. CA 19-9, D. AFP
A: CEA
Answer: A: CEA 
Explanation: Up to a half of all patients with colorectal cancer will develop liver metastases at some point and regular imaging of the liver (by ultrasound and CT scan) and measurement of carcinoembryonic antigen (CEA) is designed to diagnose this early, in order to allow curative metastectomy. Trials of the optimum follow-up pathway have suggested that CEA measurement alone can be as effective as regular imaging.Ref: Bailey and Love, 27e, page: 1266