Final answer:
The first-line intervention for a patient with multiple myeloma experiencing lower back pain is to prescribe non-steroidal anti-inflammatory drugs (NSAIDs).
Step-by-step explanation:
The first-line intervention for a patient with multiple myeloma experiencing lower back pain would be to prescribe non-steroidal anti-inflammatory drugs (NSAIDs). NSAIDs can help reduce inflammation and relieve pain. Other interventions such as corticosteroids or physical therapy may also be used depending on the individual's specific needs and response to treatment.