**Core Concept**
The question tests the clinical correlation between a scrotal mass and its possible primary site of origin, specifically exploring the association between renal cell carcinoma (RCC) and metastatic scrotal masses.

**Why the Correct Answer is Right**
Renal cell carcinoma (RCC) can metastasize to the scrotum, a phenomenon known as paraneoplastic scrotal metastasis. This occurs when cancer cells from the primary site (in this case, the kidney) spread to the scrotum via the bloodstream or lymphatic system. The absence of alpha-fetoprotein (AFP) and lactate dehydrogenase (LDH) levels in the patient's blood suggests that the primary tumor is less likely to be a germ cell tumor, such as a seminoma. Furthermore, the patient's 35-pack-year history of smoking increases his risk for RCC.

**Why Each Wrong Option is Incorrect**
**Option A:** Seminoma is a type of germ cell tumor that typically presents with elevated AFP levels, which are not present in this patient. Seminomas are also more common in younger males.
**Option C:** Epididymitis is an inflammation of the epididymis, usually caused by bacterial infection, and is not associated with a painless scrotal mass in a 49-year-old male with a history of smoking.
**Option D:** Lung cancer can metastasize to the scrotum, but the patient's presentation is more suggestive of RCC, given the microscopic hematuria and absence of AFP and LDH.

**Clinical Pearl / High-Yield Fact**
When evaluating a scrotal mass, it is essential to consider the possibility of paraneoplastic metastasis from a primary site, such as RCC, particularly in patients with a history of smoking and microscopic hematuria.

**✓ Correct Answer: B. Renal cell ca**