What is the most likely cause of high serum alkaline phosphatase and lytic lesions in a 55-year-old man with back pain?

The most likely cause of high serum alkaline phosphatase and lytic lesions in a 55-year-old man with back pain is hypervascular bone. In conditions such as metastatic disease or primary bone tumors, increased osteoblastic activity often leads to elevated levels of alkaline phosphatase in the serum. Lytic lesions indicate areas of bone resorption, which can coexist with hypervascularity due to the increased blood supply typically associated with tumor proliferation.

In the context of back pain alongside these lab findings, hypervascular bone suggests that tumor-associated osteogenesis is possibly causing both the resorption of bone (leading to lytic lesions) and elevated alkaline phosphatase levels. This pattern is particularly notable in certain malignancies, such as multiple myeloma or metastatic disease to the bone, where both lytic lesions and increased bone turnover can be present.

Conditions such as chronic soft tissue degeneration, osteolytic infectious processes, and soft tissue mass effects may lead to back pain, but these would not typically account for the combination of significantly elevated alkaline phosphatase levels and lytic lesions observed in this scenario.