Lumbar Radiculopathy: This condition, often due to a herniated disk, can cause pain that radiates from the lower back down to the ankle, typically following a dermatomal distribution. The pain can be cramping in nature and is usually unilateral.

Peripheral Artery Disease (PAD): PAD can cause intermittent claudication, which is characterized by cramping pain in the legs, typically in the calf, that occurs during walking and is relieved by rest. The pain can start in the lower back and radiate down to the ankle.
Musculoskeletal Strain: Strains in the lower back or leg muscles can cause cramping pain that radiates down to the ankle, especially if the strain involves muscles that run from the back down to the lower extremities.
Sciatica: Similar to lumbar radiculopathy, sciatica involves pain radiating from the lower back down the leg, but it is more of a syndrome than a specific diagnosis. The pain can be cramping and usually affects one side of the body.

Abdominal Aortic Aneurysm (AAA): Although AAA typically presents with abdominal pain, it can also cause back pain that radiates down to the legs if it becomes large enough to compress or involve nearby nerves. Missing this diagnosis can be fatal.
Spinal Epidural Abscess: This is a serious condition that can cause back pain radiating down the legs, along with fever, weakness, and other neurological deficits. Prompt diagnosis and treatment are crucial to prevent serious complications.
Cauda Equina Syndrome: This condition, often due to a large herniated disk, can cause severe back pain, sciatica, and neurological symptoms like bladder and bowel dysfunction. It is a medical emergency requiring immediate attention.

Piriformis Syndrome: This involves the piriformis muscle compressing the sciatic nerve, causing pain that radiates from the buttocks down to the leg and ankle. It's less common than other causes of sciatica but should be considered in the differential.
Neoplastic Processes: Tumors affecting the spine, nerves, or surrounding tissues can cause pain that radiates from the lower back down to the ankle. These are less common but important to consider, especially in patients with risk factors for cancer.
Inflammatory Arthropathies: Certain inflammatory conditions, like ankylosing spondylitis, can cause back pain that radiates down the legs, although this is less typical for these conditions.