Guillain-Barré Syndrome (GBS): This is the most likely diagnosis given the context of the question. GBS is an autoimmune disorder where the body's immune system mistakenly attacks part of its peripheral nervous system, leading to muscle weakness, and sometimes paralysis, among other symptoms. The diagnosis is often considered in patients with rapidly progressing muscle weakness.

Miller Fisher Syndrome: A rare, acquired nerve disease that is considered a variant of GBS. It is characterized by ophthalmoplegia (weakness or paralysis of the eye muscles), ataxia (lack of coordination), and areflexia (absence of reflexes).
Chronic Inflammatory Demyelinating Polyneuropathy (CIDP): An autoimmune disorder where the immune system attacks the myelin sheath covering nerve roots and peripheral nerves, leading to nerve damage and muscle weakness. It has a more gradual onset compared to GBS.
Acute Myelitis: Inflammation of the spinal cord, which can cause symptoms similar to GBS, including muscle weakness, but typically involves sensory symptoms and bladder/bowel dysfunction.

Spinal Cord Compression: A medical emergency that can present with rapidly progressing muscle weakness, similar to GBS. Causes include trauma, tumors, and abscesses.
Botulism: A rare but potentially life-threatening illness caused by a toxin produced by the bacteria Clostridium botulinum. It can cause muscle weakness, paralysis, and respiratory issues.
Myasthenia Gravis: An autoimmune disease that leads to fluctuating muscle weakness and fatigue. While it typically affects the muscles controlling eye and eyelid movement, swallowing, and breathing, it can sometimes mimic GBS.
Poliomyelitis: Although rare in areas with widespread vaccination, polio can cause acute flaccid paralysis and must be considered, especially in patients with recent travel history to endemic areas or in the context of vaccine-associated polio.

Porphyric Neuropathy: A group of disorders that result from a buildup of natural chemicals that produce porphyrin in the body. Some types can cause neurological symptoms, including muscle weakness.
Diphtheritic Neuropathy: A complication of diphtheria infection, which can cause nerve damage leading to muscle weakness.
Tick Paralysis: A rare condition caused by a toxin in the saliva of certain ticks, leading to ascending paralysis that can mimic GBS.
Heavy Metal Poisoning (e.g., Lead, Arsenic): Exposure to certain heavy metals can cause neurological symptoms, including muscle weakness, although the presentation is usually different from GBS.