## **Core Concept**
The patient's presentation of night pain and a bone lesion in the lumbar spine with biopsy suggestive of blastic skeletal metastasis indicates a need to identify the primary source of the metastasis. Blastic metastases are typically associated with certain types of cancers, and specific tumor markers can help in diagnosing the primary site.

## **Why the Correct Answer is Right**
The correct answer, **Prostate Specific Antigen (PSA)**, is appropriate because blastic skeletal metastases are commonly associated with prostate cancer. Prostate cancer often metastasizes to bones and can cause osteoblastic (blastic) lesions. PSA is a specific tumor marker for prostate cancer, making it a useful test in this clinical scenario to help identify the primary source of the metastasis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **Carcinoembryonic Antigen (CEA)** is a tumor marker, it is more commonly associated with colorectal cancer and other epithelial cancers. It is not specific for prostate cancer and would not be the first choice for investigating blastic bone metastases.
- **Option B:** **Alpha-fetoprotein (AFP)** is primarily associated with hepatocellular carcinoma and certain germ cell tumors, not typically with prostate cancer or blastic bone metastases.
- **Option D:** **CA 125** is a marker often used in the monitoring of ovarian cancer. It is not specifically associated with prostate cancer or blastic skeletal metastases.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that when encountering blastic bone metastases, especially in men, prostate cancer should be a primary consideration. The measurement of **PSA** levels is a critical step in the diagnostic workup for such patients, as it can help confirm prostate cancer as the primary source.

## **Correct Answer:** . **Prostate Specific Antigen (PSA)**