**Core Concept**
Lung to lung metastasis, also known as intrapulmonary metastasis, refers to the spread of cancer cells from one part of the lung to another, or even from one lung to the other. This phenomenon is observed in certain types of cancer that have a high propensity for metastasis.

**Why the Correct Answer is Right**
Lung to lung metastasis typically occurs in cancers that have a high incidence of hematogenous spread, which involves the spread of cancer cells through the bloodstream. In the case of lung cancer, metastasis to the contralateral lung is a known phenomenon, especially in non-small cell lung cancer (NSCLC) and small cell lung cancer (SCLC). The cancer cells can enter the bloodstream through the pulmonary veins and travel to the other lung, where they can establish new colonies.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the question, as it does not specify a type of cancer or a mechanism of metastasis.
* **Option B:** This option is incorrect because it refers to a different type of metastasis, known as lymphatic spread, which involves the spread of cancer cells through the lymphatic system, not the bloodstream.
* **Option C:** This option is incorrect because it refers to a different type of cancer that is not typically associated with lung to lung metastasis.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with lung cancer, it is essential to consider the possibility of lung to lung metastasis, especially if the patient has a history of smoking or exposure to carcinogens. Imaging studies, such as CT scans or PET scans, can help identify metastatic disease in the contralateral lung.

**Correct Answer: D.**