**Core Concept**
The question is testing knowledge of the most common cause of bone malignancy, which is a key concept in orthopedic oncology. Bone malignancies can arise from primary bone tumors or metastatic lesions from other cancers.

**Why the Correct Answer is Right**
The most common cause of bone malignancy is metastatic disease from other cancers. This is due to the fact that many cancers, such as breast, lung, and prostate cancer, have a high propensity to metastasize to the bone. The bone microenvironment provides a fertile ground for tumor cell growth and metastasis, making it a common site for cancer spread.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while primary bone tumors are a significant cause of bone malignancies, they are not the most common cause.
**Option B:** This option is incorrect because although certain infections, such as osteomyelitis, can cause bone destruction, they are not typically considered malignancies.
**Option C:** This option is incorrect because while radiation exposure can increase the risk of bone malignancies, it is not the most common cause.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that bone metastases are a common cause of bone malignancies and can be a sign of advanced cancer. A high index of suspicion is necessary to diagnose and manage bone metastases promptly.

**Correct Answer: D. Metastatic disease from other cancers**