**Core Concept:**
Tumor markers are biochemical substances produced by, or associated with, specific tumors or pathological processes. These can be detected in body fluids and serve as indirect indicators of tumor burden or activity. In the context of prostate cancer, prostate-specific antigen (PSA) is a widely used tumor marker.

**Why the Correct Answer is Right:**
Prostate-specific antigen (PSA) is a protein produced by the prostate gland. In patients with prostate cancer, the tumor can cause an increase in PSA production. Monitoring PSA levels can help assess the effectiveness of treatment and detect tumor recurrence. In this case, the patient has prostate cancer and is experiencing symptoms which may indicate disease relapse, making PSA an appropriate tumor marker to consider.

**Why Each Wrong Option is Incorrect:**
A. Prostate-specific antigen (PSA) is the correct answer, so all other options are incorrect.
B. Prolactin is a hormone involved in lactation and does not play a role in prostate cancer monitoring.
C. Carcinoembryonic antigen (CEA) is a tumor marker that can be elevated in gastrointestinal cancers, but not typically in prostate cancer.
D. Alpha-fetoprotein (AFP) is a marker for liver and gastrointestinal cancers, not prostate cancer.

**Clinical Pearl / High-Yield Fact:**
Monitoring PSA levels is essential for assessing prostate cancer treatment response and detecting recurrence. PSA serves as a valuable tool in guiding clinical decisions and adjusting treatment strategies accordingly. However, it is essential to consider other factors when interpreting PSA results, as elevated PSA levels can be influenced by factors other than cancer, such as inflammation or surgery. A rising or steadily increasing PSA level over time is more concerning for disease progression and may warrant further investigation, including imaging and biopsy. A sudden or significant decrease in PSA level following treatment indicates a successful response to therapy, while a stable or slightly elevated PSA level does not necessarily imply disease recurrence but may warrant further assessment.