Herniated Lumbar Disc: This condition is a common cause of low back pain that radiates to the posterior thigh, especially when the pain worsens in certain positions such as lying flat. The mechanism involves nerve root compression or irritation due to the herniated disc, leading to pain and discomfort.

Lumbar Degenerative Disc Disease: This condition involves the wear and tear of the spinal discs, which can cause low back pain and radiating pain to the thighs due to the loss of disc height and subsequent nerve root irritation or compression.
Lumbar Spinal Stenosis: Narrowing of the spinal canal can compress the nerves, leading to pain in the low back and thighs, especially when standing or walking, but can also be exacerbated by certain positions.
Piriformis Syndrome: The piriformis muscle can compress the sciatic nerve, causing pain in the low back and posterior thigh, which may worsen with certain positions or activities.

Cauda Equina Syndrome: A serious condition where the nerves in the spinal canal are compressed, leading to severe pain, numbness, and potentially bladder and bowel dysfunction. It is a medical emergency requiring prompt intervention.
Spinal Infection (e.g., Discitis, Osteomyelitis): Infections of the spine can cause severe back pain and systemic symptoms such as fever. Early diagnosis and treatment are crucial to prevent serious complications.
Spinal Tumor: Tumors in the spine can compress nerves, leading to pain and neurological symptoms. Although rare, missing this diagnosis can have severe consequences.

Tarlov Cyst: A rare condition where cysts form on the nerve roots in the spine, potentially causing low back and leg pain due to nerve compression.
Arachnoiditis: Inflammation of the membranes surrounding the spinal cord, which can cause severe pain and neurological symptoms. It is a rare condition but should be considered in the differential diagnosis of chronic back pain.