Stroke: The most common cause of unilateral lower limb weakness, resulting from ischemia or hemorrhage in the brain, typically affecting the motor cortex or corticospinal tract.

Peripheral neuropathy: Conditions such as sciatic neuropathy or peroneal neuropathy can cause unilateral lower limb weakness due to damage to the peripheral nerves.
Lumbar radiculopathy: Compression or irritation of the lumbar nerve roots (e.g., L4 or L5) can lead to unilateral lower limb weakness, often accompanied by pain or sensory changes.
Musculoskeletal disorders: Conditions like hip or knee osteoarthritis, or muscle injuries, can cause unilateral lower limb weakness, although this is typically more gradual in onset.

Spinal cord compression: A medical emergency that can cause unilateral or bilateral lower limb weakness, depending on the level and extent of compression. Prompt diagnosis and treatment are crucial to prevent permanent damage.
Guillain-Barré syndrome: An autoimmune disorder that can cause ascending paralysis, starting with lower limb weakness, which can progress to respiratory failure if not recognized and treated promptly.
Intracranial tumors: Although less common, tumors affecting the motor cortex or corticospinal tract can cause unilateral lower limb weakness and require urgent attention.

Inflammatory myopathies: Conditions like polymyositis or dermatomyositis can cause progressive muscle weakness, including unilateral lower limb weakness, although this is less common.
Neurodegenerative diseases: Rare conditions such as amyotrophic lateral sclerosis (ALS) or primary lateral sclerosis (PLS) can cause unilateral lower limb weakness as part of a broader neurological decline.
Vasculitis: Certain types of vasculitis, like giant cell arteritis or polyarteritis nodosa, can cause unilateral lower limb weakness due to inflammation of blood vessels supplying the nerves or muscles.