Tetanus: This condition is characterized by muscle stiffness and spasms, often starting in the jaw (lockjaw) and progressing to other parts of the body. The progressive nature of the symptoms aligns with tetanus, especially if there's a history of a wound or cut that could have introduced the tetanus bacterium.

Parkinson's Disease: While primarily known for tremors, Parkinson's can also cause rigidity and stiffness, which can progress over time. However, the presence of significant spasm might be less typical.
Multiple Sclerosis: This autoimmune disease can cause a wide range of neurological symptoms, including muscle spasms and stiffness, due to the damage of the myelin sheath around nerve fibers.
Stiff Person Syndrome: A rare autoimmune disorder but more likely than some other conditions on this list, characterized by progressive stiffness and rigidity of the muscles, often starting in the legs and progressing to other parts of the body.

Meningitis: Although meningitis primarily presents with symptoms like fever, headache, and neck stiffness, in some cases, especially in the early stages or in certain types (like tuberculous meningitis), it can present with progressive muscular rigidity and spasms. Missing this diagnosis could be fatal.
Rabies: In its later stages, rabies can cause hydrophobia (fear of water) due to spasms in the throat and laryngeal muscles when attempting to drink, along with generalized muscle spasms. Given its almost invariably fatal outcome if not treated promptly after exposure, it's crucial not to miss.
Botulism: Caused by a toxin that affects the nervous system, botulism can lead to muscle weakness, paralysis, and in some cases, rigidity and spasms, particularly if the toxin affects the cranial nerves and the muscles of respiration.

Neuromyelitis Optica (NMO): An autoimmune disease that affects the optic nerve and spinal cord, leading to vision loss and spinal cord damage. While less common, it can cause muscle spasms among its wide range of neurological symptoms.
** Isaacs' Syndrome (Acquired Neuromyotonia)**: A rare condition characterized by muscle stiffness, cramps, and delayed relaxation of the muscles after contraction, due to hyperexcitability of the motor nerves.
Hypocalcemia: Low calcium levels can cause muscle cramps, spasms, and tetany (involuntary muscle contraction), though this would typically be identified through laboratory tests rather than clinical presentation alone.