**Core Concept**
The mediastinoscope is a medical instrument used for visualizing the mediastinum, a central compartment of the thoracic cavity containing lymph nodes, nerves, and other vital structures. The instrument allows for direct visualization and biopsy of mediastinal lymph nodes, which are crucial for diagnosing and staging various cancers, including lung cancer.

**Why the Correct Answer is Right**
Mediastinoscopy typically involves the insertion of a mediastinoscope through an incision in the neck, allowing the visualization of lymph nodes in the anterior, middle, and posterior mediastinum. However, certain lymph node stations are not accessible via this approach due to their location or the presence of vital structures. For instance, the subcarinal lymph nodes (station 7) are located below the carina, a critical area where the right and left main bronchi diverge, making them difficult to access via mediastinoscopy.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided.
**Option B:** This option is not provided.
**Option C:** This option is not provided.
**Option D:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
The subcarinal lymph nodes are a critical area for lung cancer staging, and their involvement often necessitates more extensive surgical resection or other treatment modalities.

**Correct Answer: D. Subcarinal lymph nodes.**