**Core Concept**
The patient presents with a testicular hypoechoic mass and back pain, suggesting a possible testicular malignancy with metastasis to the retroperitoneal lymph nodes. The best tumor marker for this presentation would be one that is commonly elevated in testicular cancer, particularly non-seminomatous germ cell tumors (NSGCTs) and seminomas.

**Why the Correct Answer is Right**
Beta-HCG is a tumor marker that is often elevated in NSGCTs, including embryonal carcinoma, choriocarcinoma, and yolk sac tumors. It is also elevated in some seminomas, particularly those with choriocarcinomatous elements. Elevated beta-HCG levels can indicate the presence of metastatic disease, guiding further evaluation and treatment. In contrast to alpha-fetoprotein (AFP), which is elevated in yolk sac tumors and some embryonal carcinomas, beta-HCG is more specific for NSGCTs and choriocarcinomas.

**Why Each Wrong Option is Incorrect**
**Option A:** Alpha-fetoprotein (AFP) is elevated in yolk sac tumors and some embryonal carcinomas, but not as commonly in seminomas or choriocarcinomas, making it less specific for this presentation.
**Option B:** Lactate dehydrogenase (LDH) is a nonspecific marker that can be elevated in various conditions, including testicular cancer, but is not as specific for this diagnosis as beta-HCG.
**Option C:** Prostate-specific antigen (PSA) is a marker for prostate cancer, not testicular cancer, and is not relevant to this presentation.
**Option D:** Human chorionic gonadotropin (hCG) is a precursor to beta-HCG, but the more specific and commonly measured marker in clinical practice is beta-HCG.

**Clinical Pearl / High-Yield Fact**
In patients with testicular cancer, it is essential to measure both beta-HCG and AFP levels, as elevated levels can guide the diagnosis and management of metastatic disease. Additionally, serial measurements of tumor markers can help monitor response to treatment and detect recurrence.

**Correct Answer: B. Beta-HCG**