**Core Concept**
The underlying principle being tested is the diagnosis of liver masses, specifically focusing on the role of serum markers in identifying the nature of the mass. In this context, the patient's presentation suggests a possible liver tumor, and serum markers can help differentiate between primary and metastatic liver lesions.

**Why the Correct Answer is Right**
Given the patient's age, history of alcoholism, and the presence of a large liver mass, the most likely diagnosis is a hepatocellular carcinoma (HCC). The serum marker most closely associated with HCC is **alpha-fetoprotein (AFP)**. Elevated levels of AFP can be found in the majority of patients with HCC, making it a useful diagnostic tool.

**Why Each Wrong Option is Incorrect**
**Option A:** CA 19-9 is more commonly associated with pancreatic cancer.
**Option B:** CEA is often elevated in colorectal cancer.
**Option D:** PSA is specific to prostate cancer.

**Clinical Pearl / High-Yield Fact**
In patients with cirrhosis or a high risk of HCC, monitoring AFP levels can aid in early detection.

**Correct Answer:** D. Alpha-fetoprotein.