**Core Concept**
Exocrine pancreatic cancer, also known as pancreatic ductal adenocarcinoma, is a type of cancer that arises from the exocrine glands of the pancreas. Elevated tumour markers are often used in the diagnosis and monitoring of this cancer.

**Why the Correct Answer is Right**
CA-19-9 is a carbohydrate antigen that is commonly elevated in patients with exocrine pancreatic cancer. This is because many pancreatic cancer cells produce CA-19-9 as a result of the abnormal expression of the Lewis blood group antigens, which are normally present on the surface of epithelial cells. The elevation of CA-19-9 is thought to be due to the abnormal glycosylation of mucins, which are proteins that line the surface of epithelial cells. High levels of CA-19-9 have been correlated with poor prognosis and are often used as a marker for pancreatic cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** CA-125 is a tumour marker that is commonly elevated in ovarian cancer, not exocrine pancreatic cancer.
**Option B:** CEA is a tumour marker that is elevated in a variety of cancers, including colorectal, lung, and breast cancer, but it is not the most sensitive marker for exocrine pancreatic cancer.
**Option C:** CA-15-3 is a tumour marker that is primarily used to monitor breast cancer, and it is not commonly elevated in exocrine pancreatic cancer.

**Clinical Pearl / High-Yield Fact**
CA-19-9 is not specific to pancreatic cancer, as it can also be elevated in other conditions such as chronic pancreatitis, gallbladder disease, and bile duct obstruction. However, a high level of CA-19-9 in a patient with pancreatic cancer suggests a poor prognosis and may indicate a need for more aggressive treatment.

**✓ Correct Answer: D. CA-19-9**