**Core Concept**
Hepatocellular carcinoma (HCC) is a primary malignancy of the liver, and tumor markers play a crucial role in its diagnosis and monitoring. Tumor markers are substances produced by cancer cells or in response to cancer that can be detected in the blood or other bodily fluids.

**Why the Correct Answer is Right**
Alpha-fetoprotein (AFP) is a well-established tumor marker for HCC, particularly in the context of chronic liver disease and cirrhosis. Elevated levels of AFP are often associated with HCC, especially in patients with a history of hepatitis B or C infection, alcohol abuse, or non-alcoholic fatty liver disease (NAFLD). The mechanism behind AFP's association with HCC is not fully understood but is believed to involve the abnormal expression of genes involved in embryonic development.

**Why Each Wrong Option is Incorrect**
**Option A:** HCG is primarily associated with trophoblastic diseases, such as choriocarcinoma and gestational trophoblastic disease.
**Option B:** CA 19-9 is a tumor marker for pancreatic cancer, not HCC.
**Option C:** LDH is an enzyme found in many tissues and is not specific to HCC.

**Clinical Pearl / High-Yield Fact**
AFP levels can be elevated in other conditions, such as cirrhosis, hepatitis, and pregnancy, which can lead to false positives. Therefore, a diagnosis of HCC should be made in conjunction with imaging studies and other clinical findings.

**Correct Answer: C. Alpha-fetoprotein (AFP)**