Guillain-Barré Syndrome: An autoimmune disorder that often follows a minor infection, leading to rapid onset muscle weakness, which can start in the lower legs and ascend. It's a common cause of acute flaccid paralysis and can progress rapidly, making it a prime consideration in this scenario.

Multiple Sclerosis (MS): An autoimmune disease affecting the central nervous system, which can present with a wide range of neurological symptoms, including weakness, vision problems, and difficulty with coordination and balance. The relapsing-remitting form of MS could initially present with lower leg weakness.
Chronic Inflammatory Demyelinating Polyneuropathy (CIDP): Similar to Guillain-Barré Syndrome but with a more chronic course, CIDP is an autoimmune disorder that affects the nerves, leading to progressive weakness and sometimes paralysis.
Spinal Cord Compression or Lesion: Conditions such as herniated disks, spinal stenosis, or tumors can compress the spinal cord, leading to progressive lower limb weakness.

Spinal Epidural Abscess: A serious condition that requires immediate medical attention, characterized by an abscess in the epidural space surrounding the spinal cord, which can cause progressive neurological deficits, including weakness, and can lead to paralysis or death if not promptly treated.
Cauda Equina Syndrome: A condition that occurs when the nerves in the spinal canal are compressed, leading to pain, numbness, and weakness in the lower back and legs, as well as potential bladder and bowel dysfunction. It's a medical emergency.
Neurosyphilis: A complication of untreated syphilis infection that can affect the central nervous system, leading to a variety of neurological symptoms, including weakness.

Hereditary Neuropathies (e.g., Charcot-Marie-Tooth Disease): A group of inherited disorders that affect the peripheral nerves, leading to progressive muscle weakness and atrophy, primarily in the legs.
Mitochondrial Myopathies: A group of disorders caused by mitochondrial dysfunction, affecting muscle and nerve cells, and can present with progressive muscle weakness.
Vasculitis (e.g., Polyarteritis Nodosa): Inflammation of the blood vessels that can affect various parts of the body, including the nerves, leading to weakness and other neurological symptoms.
Toxic or Metabolic Neuropathies: Certain toxins (e.g., heavy metals) or metabolic disorders (e.g., vitamin deficiencies) can cause progressive neuropathy, leading to lower leg weakness.