**Core Concept:**

The question is regarding the most common site of post-mortem findings for distant tumor spread in cancer patients, particularly in cases where the primary tumor is not evident. In such cases, the focus is on the organs that are most likely to harbor secondary tumors due to their high blood flow, rich vascularity, and close proximity to the primary cancer source. These organs are known as "sink organs" or "sink stations," as they are the last stop for cancer cells traveling through the body's lymphatic and hematic systems.

**Why the Correct Answer is Right:**

**Correct Answer: A. Lungs**

In most cases, lungs are the primary site of post-mortem findings due to the high blood flow, rich vascularity, and close proximity to the primary cancer source (thoracic organs like the lungs, pleura, or mediastinum). Cancer cells can disseminate hematogenously (through the bloodstream) or lymphogenously (through the lymphatic system) to the lungs. Once in the lungs, these cells can colonize and form secondary tumors.

**Why Each Wrong Option is Incorrect:**

**Option B. Liver**: Although the liver can be involved in cancer, it is less common than lung involvement due to its lower blood flow and vascularity compared to the lungs. However, this option can still be relevant in certain cases, such as liver cancer or primary hepatic malignancies.

**Option C. Lymph nodes**: While lymph nodes can be involved in cancer, they are not the primary site of post-mortem findings. The correct answer, lungs, and other sink organs, are more likely to demonstrate secondary tumor formation.

**Option D. Spleen**: Similarly to lymph nodes, the spleen is not the primary site of post-mortem findings. The lungs, as the sink organs, are more likely to demonstrate secondary tumor formation.

**Why Each Wrong Option is Incorrect:**

**Option E. Brain**: Involvement of the brain in cancer is typically late-stage dissemination, not a common site for post-mortem findings.

**Core Concept**: Remember that the focus in this question is on sink organs that are more likely to demonstrate secondary tumor formation due to their high blood flow, rich vascularity, and close proximity to the primary cancer source. These organs serve as a final stop for disseminated cancer cells, making them more likely to show secondary tumors at post-mortem examination.