S1 Radiculopathy due to L5-S1 Herniated Disc: The patient's symptoms of pain radiating from the low back into the buttocks and down to the calf, with numbness on the lateral and plantar surface of the foot, are classic for S1 radiculopathy. This condition is most commonly caused by a herniated disc at the L5-S1 level, which compresses the S1 nerve root.

Lumbar Degenerative Disc Disease: This condition can cause similar symptoms due to the degeneration of the discs in the lumbar spine, leading to nerve root compression.
L4-L5 Herniated Disc: Although less likely than L5-S1, a herniated disc at the L4-L5 level can also cause radiculopathy, especially if it affects the L5 nerve root, leading to pain and numbness in the foot.
Piriformis Syndrome: Compression of the sciatic nerve by the piriformis muscle can cause pain, numbness, and tingling in the buttocks and down into the leg, similar to the symptoms described.

Cauda Equina Syndrome: A serious condition that occurs when the nerves in the spinal canal are compressed, leading to severe pain, numbness, and potentially bladder and bowel dysfunction. It is a medical emergency.
Spinal Infection (e.g., Abscess or Osteomyelitis): Infections in the spine can cause severe back pain and neurological symptoms, including numbness and weakness in the legs.
Spinal Tumor: Tumors in the spine can compress nerve roots, leading to pain, numbness, and weakness.

Tarlov Cyst: A rare condition where cysts form on the nerve roots in the spine, potentially causing nerve compression and symptoms similar to those described.
Arachnoiditis: Inflammation of the arachnoid membrane surrounding the spinal cord and nerve roots, which can cause severe pain, numbness, and tingling in the legs.
Neurogenic Claudication due to Lumbar Spinal Stenosis: Narrowing of the spinal canal can cause pain, numbness, and weakness in the legs, especially with walking or standing.