Musculoskeletal strain: This is the most common cause of low back pain in adults, often resulting from overuse, heavy lifting, or sudden awkward movements. Given the age and the presentation of low back pain without additional alarming symptoms, musculoskeletal strain is a plausible initial consideration.

Degenerative disc disease: As people age, the spinal discs can degenerate, leading to pain and stiffness in the lower back. A 40-year-old woman may start to experience the effects of degenerative changes in the spine.
Herniated disc: A herniated disc can cause pain in the lower back and may radiate to the legs if it compresses a nerve root. This condition is common in adults and can be related to age, lifestyle, and physical activity.
Osteoarthritis: While more common in older adults, osteoarthritis of the spine can begin to cause symptoms in individuals in their 40s, especially if there's a history of trauma, obesity, or genetic predisposition.

Cauda equina syndrome: A serious condition that occurs when the nerves in the spinal canal are compressed, leading to symptoms such as severe low back pain, bowel or bladder dysfunction, and numbness or weakness in the legs. Prompt recognition and treatment are crucial to prevent permanent damage.
Spinal infection (e.g., osteomyelitis, discitis): Infections of the spine can cause severe back pain and are medical emergencies. Risk factors include recent travel, immunocompromised state, or history of intravenous drug use.
Spinal fracture: Especially in the context of osteoporosis or trauma, a spinal fracture can cause sudden onset of severe back pain. This is particularly concerning in postmenopausal women or those with risk factors for osteoporosis.
Cancer with spinal metastasis: Although less common, cancer that has spread to the spine can cause back pain. A thorough history, including questions about weight loss, fatigue, and family history of cancer, is essential.

Ankylosing spondylitis: A form of arthritis that primarily affects the spine, although it can also affect other joints. It's characterized by chronic inflammation of the spine and the area where the tendons and ligaments attach to the bone.
Spondylolisthesis: A condition where one of the vertebrae slips forward over the bone below it. This can cause back pain and may be more common in certain athletic populations or those with a family history.
Paget's disease of bone: A chronic condition that can result in enlarged and misshapen bones due to abnormal bone destruction and regrowth. It can affect any bone, but when it affects the spine, it can cause back pain.