Which condition is more likely if both M-wave is decreased and SNAP is normal?

When both the M-wave is decreased and the sensory nerve action potential (SNAP) is normal, it indicates that there is likely an issue in the motor system while the sensory system remains intact. The M-wave represents the electrical activity of muscle fibers in response to motor nerve stimulation, so a decrease suggests that there may be a problem with the motor pathway, which includes the motor neurons or the muscles themselves.

In this context, muscle disease, such as myopathy, would be the correct condition because it can result in diminished M-wave activity due to impaired muscle function, while sensory fibers, which are responsible for creating a normal SNAP, are unaffected. Myopathies can have a variety of causes, including genetic factors, inflammatory processes, or metabolic issues.

The normal SNAP indicates that sensory nerve function is preserved, which rules out conditions that typically would affect both sensory and motor responses concurrently, giving even more weight to the possibility of a localized issue with muscle function rather than a broader neuronal problem like peripheral neuropathy or spinal cord injury.