What is the primary diagnostic test for ALS?

• A. MRI
• B. EMG + nerve conduction
• C. Blood and urine tests
• D. Neuro exam

Answer: (B)

Diagnosing ALS relies on detecting a pattern of motor neuron involvement across multiple regions and watching it progress. The test that best confirms this pattern is electrophysiologic testing with electromyography plus nerve conduction studies. EMG shows evidence of both active denervation and chronic reinnervation in several muscles, reflecting widespread motor neuron loss. Nerve conduction studies help differentiate motor neuron disease from other problems by typically showing preserved sensory pathways and reduced motor responses due to axonal loss. This combination provides objective evidence of a motor neuron process consistent with ALS and helps rule out other conditions that can mimic it, such as compressive spine lesions or peripheral neuropathies. Imaging and lab tests mainly serve to exclude alternative diagnoses, while a neurologic exam identifies signs but does not by itself confirm the widespread motor neuron involvement.