Musculoskeletal strain or sprain: This is often the most common cause of lower back pain, especially when it's exacerbated by flexion. The inability to walk could be due to severe pain or muscle spasm.

Herniated lumbar disc: A herniated disc can cause significant pain, especially with flexion, and can lead to neurological symptoms that might impair walking.
Degenerative disc disease: This condition can cause chronic back pain and stiffness, potentially worsening with flexion and affecting mobility.
Spondylosis or spondylolisthesis: These conditions involve defects in the vertebrae and can cause back pain and neurological symptoms, especially with certain movements like flexion.

Cauda equina syndrome: Although less common, this is a medical emergency that can cause severe back pain, neurological symptoms, and potentially permanent damage if not promptly treated. It's characterized by compression of the cauda equina nerves.
Spinal infection (e.g., osteomyelitis, discitis): Infections of the spine can cause severe back pain and systemic symptoms. Early diagnosis and treatment are crucial to prevent serious complications.
Spinal fracture: Especially in patients with osteoporosis or those who have experienced trauma, a spinal fracture could cause severe back pain and mobility issues.

Spinal tumors: Both benign and malignant tumors can cause back pain and neurological symptoms. They are less common but should be considered, especially if there's a history of cancer.
Ankylosing spondylitis: A form of arthritis that can cause chronic back pain and stiffness, potentially leading to significant disability if not managed properly.
Osteonecrosis of the spine: A rare condition where the bone tissue dies due to lack of blood supply, which can cause back pain and mobility issues.