Okay, let me try to work through this question. The patient is a 54-year-old woman with abdominal fullness, early satiety, and a history of constipation. The radiographic finding is an "apple core" lesion in the sigmoid colon. The question is asking which marker would be elevated here.

First, I need to connect the symptoms and imaging to a specific condition. The "apple core" lesion is a classic radiographic sign of a colonic mass, particularly a tumor that causes a narrowing of the bowel lumen. This is often seen in colorectal cancer. The symptoms of abdominal fullness and early satiety might be due to a mass in the sigmoid colon causing partial obstruction or affecting the gastrointestinal tract's motility.

Now, the next step is to think about tumor markers associated with colorectal cancer. The most common ones are carcinoembryonic antigen (CEA) and CA 19-9. CEA is frequently elevated in colorectal adenocarcinomas. CA 19-9 is also used but more commonly associated with pancreatic cancer. However, in the context of colorectal cancer, CEA is the primary marker. Other markers like CA 125 or AFP are not typically associated with colorectal cancer. CA 125 is more for ovarian cancer, AFP for hepatocellular carcinoma or germ cell tumors.

Looking at the options, the correct answer should be CEA. The other options (like CA 19-9, CA 125, AFP) are not the primary markers here. The "apple core" lesion is a key clue for a colonic tumor, and CEA elevation is expected. So the answer is CEA. Let me check if there's any other possibility. Maybe if there's a question about neuroendocrine tumors, but that's not the case here. The clinical presentation and imaging point towards a colorectal adenocarcinoma, so CEA is the marker.

**Core Concept**
The "apple core" lesion is a classic imaging finding of **colorectal adenocarcinoma**, particularly in the sigmoid colon. **Carcinoembryonic antigen (CEA)** is a tumor marker commonly elevated in colorectal cancer due to its expression in gastrointestinal tract tumors.

**Why the Correct Answer is Right**
Colorectal adenocarcinomas often overexpress **CEA**, a glycoprotein antigen produced by embryonic gastrointestinal tissue and re-expressed in malignant conditions. Elevated CEA levels correlate with tumor burden and are used for monitoring response to therapy. The "apple core" lesion (a circumferential narrowing of the bowel lumen) is pathognomonic for **colorectal carcinoma**, making CEA the most specific marker for this clinical scenario.

**Why Each Wrong Option is Incorrect**
**Option A:** **CA 19-9** is primarily elevated in **pancreatic adenocarcinoma** and some biliary tract cancers, not colorectal.
**Option B:** **CA 125** is a marker for **ovarian cancer** and peritoneal carcinomatosis.
**Option D:** **Alpha-fetoprotein (AFP)** is elevated in **hepatocellular carcinoma** and germ cell tumors, not colorectal cancer.

**Clinical Pearl / High-Yield Fact**
**CEA

✓ Correct Answer: B. Carcinoembryonic antigen (CEA)