**Core Concept**
Carcinoma embryonic antigen (CEA) is a tumor marker used primarily to monitor the progression or recurrence of certain types of cancers, particularly colorectal cancer. Elevated CEA levels may also be seen in other cancers, such as lung, breast, and gastric cancer. However, CEA is not a specific marker for any particular cancer, and its elevation can be influenced by various factors.

**Why the Correct Answer is Right**
CEA is a mucin glycoprotein produced by embryonic colonic epithelium and certain tumor cells. Elevated CEA levels in cancer patients are thought to reflect the presence of tumor cells that produce this antigen. However, CEA is not a specific marker for cancer and can be elevated in other conditions, such as smoking, inflammatory bowel disease, and certain infections. The correct answer is a condition where CEA is not typically elevated.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because CEA can be elevated in certain types of lung cancer, such as adenocarcinoma and squamous cell carcinoma.
**Option B:** This option is incorrect because CEA can be elevated in certain types of breast cancer, particularly in cases where there is a mucinous or colloid component.
**Option C:** This option is incorrect because CEA can be elevated in certain types of gastric cancer, particularly in cases where there is a mucinous or signet-ring cell component.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that CEA levels can be influenced by various factors, including smoking, inflammatory bowel disease, and certain infections. Therefore, a single elevated CEA level does not necessarily indicate cancer.

**Correct Answer: D.**