**Core Concept**
Lung to lung metastasis involves the spread of cancer cells from one lung to the other through the bloodstream. This phenomenon is more common in certain types of cancer, particularly those originating from the lung itself or other primary sites that are known to metastasize hematogenously.

**Why the Correct Answer is Right**
The correct answer is related to the concept of hematogenous spread, where cancer cells are carried through the bloodstream to distant sites. In the case of lung to lung metastasis, the cancer cells are likely to originate from a primary site that has a high propensity for hematogenous spread. One such example is small cell carcinoma of the lung, which is known to metastasize rapidly through the bloodstream. This type of cancer often spreads to the contralateral lung, making lung to lung metastasis a notable feature of this disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specifically address the concept of lung to lung metastasis. While certain types of cancer may metastasize to the lungs, this option does not provide a clear link to the phenomenon of lung to lung spread.

**Option B:** This option is incorrect because it refers to a different mechanism of metastasis. While direct extension and lymphatic spread are important pathways for cancer metastasis, they are not directly related to the concept of lung to lung metastasis.

**Option C:** This option is incorrect because it is too broad and does not specifically address the phenomenon of lung to lung metastasis. While certain types of cancer may metastasize to the lungs, this option does not provide a clear link to the concept of lung to lung spread.

**Clinical Pearl / High-Yield Fact**
One notable example of lung to lung metastasis is seen in small cell carcinoma of the lung, which has a high propensity for hematogenous spread and often presents with bilateral lung involvement.

**Correct Answer:** B. Small cell carcinoma of the lung.