Spondylolisthesis or Spondylosis with associated Lumbar Spinal Stenosis: The patient's long history of low back pain, significant nocturnal pain, and relief with spine flexion are classic symptoms for lumbar spinal stenosis. The presence of psoriatic arthritis increases the risk for spondylolisthesis or spondylosis, conditions that can lead to spinal stenosis. Numbness in the bilateral anterior lateral thighs and toes also supports this diagnosis, as it suggests nerve root compression.

Degenerative Disc Disease: This condition can cause chronic low back pain and is common in individuals with a long history of back pain. However, the specific pattern of relief with flexion and the distribution of numbness might be less typical.
Lumbar Disc Herniation: Could explain the nocturnal pain and numbness, especially if the herniation is compressing specific nerve roots. However, the relief with flexion is more characteristic of spinal stenosis.
Psoriatic Spondylitis: Given the patient's history of psoriatic arthritis, inflammation of the spine (spondylitis) could be a contributing factor to the back pain and could potentially lead to structural changes over time.

Cauda Equina Syndrome: Although less likely, this is a medical emergency that requires immediate attention. Symptoms can include severe low back pain, bilateral sciatica, numbness in the "saddle" area, and potentially bladder or bowel dysfunction. The presence of significant nocturnal pain and bilateral lower limb numbness necessitates consideration of this diagnosis.
Spinal Infection (e.g., Discitis or Osteomyelitis): Infections of the spine can present with severe back pain, especially at night, and can be a complication in patients with chronic conditions like psoriatic arthritis. Fever, weight loss, or other systemic symptoms might also be present.
Malignancy (Metastatic or Primary Spinal Tumors): Cancer can cause back pain that is worse at night and can lead to neurological symptoms due to compression of the spinal cord or nerve roots. A history of cancer or unexplained weight loss would increase the suspicion for this diagnosis.

Arachnoiditis: Inflammation of the arachnoid membrane surrounding the spinal cord, which can cause severe back and leg pain, as well as numbness. This condition is relatively rare and often associated with previous spinal surgery, infection, or contrast dye used in myelograms.
Ehlers-Danlos Syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. Although rare, it could potentially contribute to spinal instability or other musculoskeletal issues that might explain some of the patient's symptoms.