What is the likely cause of low back pain in a patient with no history of trauma and muscle spasm upon examination?

Mechanical low back pain is the most likely cause in this scenario due to the absence of trauma and the presence of muscle spasm upon examination. Mechanical low back pain typically arises from issues related to the spine, muscles, ligaments, or intervertebral discs and often manifests in patients without an identifiable acute injury. The characteristic muscle spasm can indicate the body's response to pain or stress in the lumbar region, suggesting an underlying mechanical issue rather than a specific injury or structural anomaly.

While muscle strain could also be a potential cause, it typically involves an identifiable event or overexertion, which is not present in this case. Herniated discs and radicular pain are usually associated with more specific neurological symptoms or a history of trauma, which the patient lacks. Thus, with no prior trauma and evidence of muscle spasm, mechanical low back pain provides the most fitting explanation for the patient's symptoms.