Final answer:
The most likely cause of low back pain with paravertebral spasm after lifting a heavy object is muscle strain, often due to improper lifting technique, which can lead to muscle overstretching or tearing.
Step-by-step explanation:
The most likely cause of low back pain with paravertebral spasm after lifting a heavy object is muscle strain.
When an individual lifts a heavy object, particularly if proper lifting techniques are not used, the muscles of the lower back can be overstretched or torn. This muscle strain leads to pain and can be accompanied by paravertebral muscle spasms as a reflex reaction to stabilize the area and prevent further injury. Strains are more immediate after such incidents, whereas other conditions like herniated discs, spinal stenosis, and sciatica often develop over time or due to specific structural changes in the spine.
However, lifting heavy objects improperly can exacerbate existing conditions or even cause disc damage that turns into herniation, spurring symptoms of sciatica or contributing to spinal stenosis over time. Given the acute scenario described, muscle strain is most typical.