Musculoskeletal Strain: This is the most common cause of lower back pain, often resulting from overuse, heavy lifting, or sudden movement. The pain is typically localized to the lower back and can be unilateral, as in this case, affecting the left side.

Lumbar Disc Herniation: Herniation of the lumbar discs can cause pain, numbness, and weakness in the lower back and radiate down to the legs. The pain can be unilateral and is often exacerbated by movement or certain positions.
Degenerative Disc Disease: Wear and tear on the spinal discs can lead to lower back pain, which may be more pronounced on one side. This condition is common in older adults and can be associated with stiffness and limited mobility.
Sacroiliitis: Inflammation of the sacroiliac joints can cause lower back pain, often on one side, and may radiate down to the buttocks or thighs. This condition can be associated with inflammatory back pain and stiffness.

Kidney Stone: A kidney stone can cause severe, unilateral lower back pain that may radiate to the groin. It's crucial to consider this diagnosis due to the potential for severe complications if not promptly treated.
Pyelonephritis: An infection of the kidney can cause flank pain, which may be perceived as lower back pain, along with symptoms like fever, nausea, and dysuria. Early diagnosis and treatment are essential to prevent complications.
Aortic Aneurysm: An abdominal aortic aneurysm can cause lower back pain, especially if it is leaking or ruptured. This is a medical emergency requiring immediate attention.
Osteoporotic Compression Fracture: Weakened bones can lead to vertebral compression fractures, causing sudden onset of back pain. This condition is particularly concerning in older adults or those with known osteoporosis.

Spinal Tumor: Tumors affecting the spine can cause progressive lower back pain, which may be worse at night or with activity. While rare, this diagnosis is critical due to its potential impact on prognosis and treatment.
Inflammatory Spondyloarthropathies: Conditions like ankylosing spondylitis can cause chronic lower back pain and stiffness, often with an inflammatory component. These diagnoses are less common but important to consider in patients with persistent symptoms.
Referred Pain from Intra-abdominal Pathology: Certain abdominal conditions, such as pancreatitis or a ruptured ovarian cyst, can refer pain to the lower back. While less common, these conditions can be serious and require prompt medical evaluation.