Lumbar Radiculopathy (L4-L5 or L5-S1 Disc Herniation): This is the most common cause of low back pain radiating into the posterior thigh, typically due to a herniated disc compressing a nerve root. The pain pattern and distribution depend on the specific nerve root involved.

Lumbar Strain: Muscle strain in the lower back can cause pain that radiates into the posterior thigh, especially if the strain involves the muscles that attach to the pelvis or femur.
Piriformis Syndrome: The piriformis muscle runs from the base of the spine to the thighbone, and when it compresses the sciatic nerve, it can cause pain in the buttocks and posterior thigh.
Sacroiliac Joint Dysfunction: The sacroiliac joints connect the sacrum to the ilium bones, and dysfunction here can cause pain in the lower back and radiate into the posterior thigh.

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 requires immediate medical attention.
Spinal Infection (Abscess or Osteomyelitis): Infections of the spine can cause severe back pain and neurological symptoms, including radiating pain into the thigh.
Spinal Tumor: Tumors in the spine can compress nerves, causing back pain and radiating pain into the thigh.

Tarlov Cyst: A rare condition where fluid-filled cysts form in the nerve roots of the spine, potentially causing pain and neurological symptoms.
Arterial Disease (Aortoiliac Occlusive Disease): While more common in the legs, severe arterial disease can cause pain in the buttocks and thighs, especially with exertion.
Referred Pain from Abdominal or Pelvic Organs: In rare cases, pain from abdominal or pelvic organs (e.g., kidney stones, appendicitis) can refer pain to the lower back and thigh.