**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, neck, and upper limbs. This obstruction can be caused by various factors, including neoplastic growths.

**Why the Correct Answer is Right**
Lung cancer, particularly non-small cell lung cancer (NSCLC), is the most common cause of superior vena cava syndrome. The tumor can invade or compress the superior vena cava, leading to obstruction. This is often due to the location of lung tumors in the right upper lobe or the hilar region, which can directly impinge on the superior vena cava. Additionally, lung cancer can also cause SVCS through the formation of a tumor thrombus or by metastasizing to the mediastinum, further compromising venous return.

**Why Each Wrong Option is Incorrect**
**Option B:** Lymphoma can cause SVCS, but it is less common than lung cancer. Lymphoma-related SVCS is often associated with mediastinal masses, which can compress the superior vena cava, but it is not the most common cause.

**Option C:** Metastasis can also cause SVCS, but it is typically a secondary event, resulting from the spread of cancer from another primary site. While metastatic disease can involve the superior vena cava, it is not the most common primary cause of SVCS.

**Option D:** Thyroid cancer is an uncommon cause of SVCS. While thyroid cancer can metastasize to the mediastinum, it is not typically associated with superior vena cava involvement.

**Clinical Pearl / High-Yield Fact**
In patients presenting with SVCS, it is essential to consider lung cancer as a primary diagnosis, particularly if there are symptoms suggestive of lung cancer, such as cough, hemoptysis, or weight loss.

**✓ Correct Answer: A. Lung cancer**