**Core Concept**
Superior vena cava syndrome (SVCS) is a clinical condition characterized by obstruction of the superior vena cava, leading to impaired venous return from the head and upper limbs. This obstruction is most commonly caused by external compression of the superior vena cava by a tumor.

**Why the Correct Answer is Right**
Small cell carcinoma of the lung is a highly aggressive and malignant tumor type, often presenting with advanced disease at diagnosis. One of the characteristic features of small cell carcinoma is its tendency to invade and metastasize locally, including the mediastinum and surrounding structures. As a result, small cell carcinoma can compress the superior vena cava, leading to SVCS. The rapid growth rate and aggressive behavior of small cell carcinoma make it more likely to cause SVCS compared to other tumor types.

**Why Each Wrong Option is Incorrect**
**Option A:** Lymphoma can also cause SVCS, but it is less common than small cell carcinoma. Lymphoma typically presents with systemic symptoms and may not compress the superior vena cava as frequently as small cell carcinoma.

**Option C:** Non-small cell carcinoma of the lung can also cause SVCS, but it is less aggressive than small cell carcinoma and less likely to invade and compress the superior vena cava. Non-small cell carcinoma typically presents with more localized disease and may not cause SVCS as frequently as small cell carcinoma.

**Option D:** Metastasis to the mediastinum can cause SVCS, but it is a less common cause compared to primary lung tumors like small cell carcinoma. Metastasis typically presents with multiple sites of disease and may not compress the superior vena cava as frequently as a primary tumor.

**Clinical Pearl / High-Yield Fact**
Small cell carcinoma of the lung is often associated with paraneoplastic syndromes, including Eaton-Lambert syndrome, syndrome of inappropriate antidiuretic hormone secretion (SIADH), and hypercalcemia of malignancy. These paraneoplastic syndromes can occur even in the absence of distant metastasis and are an important consideration in the diagnosis and management of small cell carcinoma.

**✓ Correct Answer: B. Small cell carcinoma**