**Core Concept**
Carcinoembryonic antigen (CEA) is a tumor marker used primarily for monitoring and follow-up of patients with colorectal cancer. CEA is a protein normally produced during embryonic development, and its levels are usually low or undetectable in adults. Elevated CEA levels are associated with various types of cancer, including colorectal, breast, lung, and others.

**Why the Correct Answer is Right**
CEA is not useful for screening for colorectal cancer (Option A) as it lacks sufficient sensitivity and specificity. It does not provide confirmative evidence of cancer (Option B), as CEA levels can be elevated in non-cancerous conditions such as inflammatory bowel disease, liver disease, or smoking. However, CEA levels are helpful for follow-up after resection (Option C) as they can indicate recurrence or metastasis. CEA levels do not decrease immediately after tumor resection (Option D), as it takes time for the tumor to regress and for CEA levels to normalize. Additionally, tumor size does not directly correlate with CEA levels (Option E), as CEA levels can be elevated in small tumors or in tumors with high CEA production.

**Why Each Wrong Option is Incorrect**
**Option A:** Screening for colorectal cancer using CEA is not recommended due to its low sensitivity and specificity.
**Option B:** CEA levels can be elevated in non-cancerous conditions, making it an unreliable marker for confirmative diagnosis.
**Option D:** CEA levels may remain elevated for some time after tumor resection due to residual tumor or metastatic disease.
**Option E:** Tumor size does not directly correlate with CEA levels, as CEA production is influenced by various factors.

**Clinical Pearl / High-Yield Fact**
CEA levels should be interpreted in the context of individual patient factors, such as the presence of liver disease or smoking, and should be used in conjunction with other diagnostic tests and clinical evaluation.

**Correct Answer: C. Helpful for follow-up after resection**