Musculoskeletal strain: This is the most common cause of low back pain, often resulting from overuse, heavy lifting, or sudden movement. The pain typically remains in the lower back and may radiate to one side, consistent with the patient's symptoms.

Lumbar facet joint syndrome: This condition involves inflammation of the facet joints in the lumbar spine, leading to pain on one side of the lower back that can radiate to the opposite side.
Sacroiliac joint dysfunction: The sacroiliac joint connects the sacrum to the ilium bone, and dysfunction here can cause low back pain that may radiate to one side.
Degenerative disc disease: Wear and tear on the spinal discs can cause low back pain, which may be unilateral and not necessarily radiate down the leg.

Abdominal aortic aneurysm: Although less common, an abdominal aortic aneurysm can present with low back pain, especially if it is expanding or leaking. This is a medical emergency.
Kidney stones or pyelonephritis: Pain from kidney stones or infection can radiate to the lower back and may be felt on one side.
Ovarian cyst or torsion: In female patients, ovarian cysts or torsion can cause severe lower abdominal or back pain that may be unilateral.

Spinal epidural abscess: A rare but serious condition where an abscess forms in the spinal epidural space, which can cause severe back pain and neurological deficits if not promptly treated.
Osteoid osteoma: A benign bone tumor that can cause nocturnal pain, which is often relieved by NSAIDs. The pain can be localized to one area of the spine.
Spinal metastasis: Cancer that has spread to the spine can cause back pain, which may be unilateral and not necessarily radiate down the leg.