**Core Concept**
Lung to lung metastasis is a rare phenomenon where a tumor originates in one lung and metastasizes to the other lung. This type of metastasis is often associated with certain histological types of lung cancer.

**Why the Correct Answer is Right**
Adenocarcinoma of the lung is the most common type of lung cancer, particularly in non-smokers. It is known to metastasize through the airways, a process known as intrapulmonary metastasis or airway seeding. This occurs when cancer cells break away from the primary tumor and are carried through the bronchi to the opposite lung, where they establish new tumors. Adenocarcinoma's propensity for airway seeding and subsequent lung to lung metastasis is thought to be related to its tendency to grow in a centriacinar or lepidic pattern, which allows for easier access to the airways.

**Why Each Wrong Option is Incorrect**

* **Option B:** Squamous cell carcinoma is a type of non-small cell lung cancer that typically arises in the central airways. While it can metastasize to the lymph nodes and other parts of the body, it is less likely to metastasize through the airways to the opposite lung compared to adenocarcinoma.
* **Option C:** Small cell carcinoma is a highly aggressive and malignant type of lung cancer that often presents with widespread metastasis at diagnosis. However, it is less likely to follow the airway seeding pathway and more likely to spread through the lymphatic and hematogenous routes.
* **Option D:** Neuroendocrine tumors of the lung, such as carcinoids and large cell neuroendocrine carcinomas, can metastasize to the lymph nodes and other parts of the body. However, they are less commonly associated with lung to lung metastasis compared to adenocarcinoma.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with lung cancer, it is essential to consider the possibility of lung to lung metastasis, particularly if the primary tumor is an adenocarcinoma. This can be an important consideration when planning treatment and making decisions about surgical intervention.

**✓ Correct Answer: A. Adenocarcinoma of lung**