Mechanical Lower Back Pain: This is the most common cause of lower thoracic and lumbar spinal pain in a young adult, especially given the physical demands of a security guard's job. Prolonged standing, lifting, or bending can strain the muscles and joints in the lower back, leading to pain.

Musculoskeletal Strain: Given the physical nature of the patient's job, a musculoskeletal strain is a plausible diagnosis. This could involve strain to the muscles or ligaments in the lower back.
Herniated Disc: A herniated disc in the lower thoracic or lumbar spine could cause pain in these areas, especially if the herniation is compressing a nerve root.
Spondylosis or Spondylolisthesis: These conditions involve defects in the pars interarticularis (a part of the vertebrae) and can lead to lower back pain, especially in younger individuals who may have a congenital predisposition or have experienced stress fractures from repetitive strain.

Cauda Equina Syndrome: Although rare, this condition is a medical emergency. It occurs when the nerves in the spinal canal are compressed, leading to symptoms such as severe lower back pain, bowel or bladder dysfunction, and numbness or weakness in the legs. Early diagnosis 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 considered medical emergencies due to the potential for serious complications, including abscess formation and spinal instability.
Spinal Tumor: Tumors in the spine, whether benign or malignant, can cause back pain and neurological symptoms. Early detection is critical for effective treatment and to prevent neurological deterioration.

Ankylosing Spondylitis: A chronic inflammatory disease that can cause pain and stiffness in the lower back, although it typically starts at a younger age and is associated with other symptoms like morning stiffness and improvement with exercise.
Osteoporotic Compression Fracture: Although more common in older individuals, osteoporotic compression fractures can occur in younger adults, especially those with risk factors such as steroid use, smoking, or a family history of osteoporosis.
Scheuermann's Kyphosis: A condition that affects the upper back but can also involve the lower thoracic spine, leading to kyphosis (a rounded appearance of the back) and back pain. It typically presents in adolescence but can be diagnosed later in life.