Suspected ALS criteria require which of the following sign patterns?

• A. LMN signs only
• B. UMN signs only
• C. LMN + UMN signs in 2 regions
• D. No signs

Answer: (A)

The key idea is that suspicion of ALS rests on evidence of lower motor neuron involvement. Lower motor neuron signs—fasciculations, weakness with muscle atrophy, decreased or absent reflexes—reflect degeneration of the anterior horn cells and peripheral nerves affected in ALS. At the earliest stage, upper motor neuron signs may be minimal or absent, so just seeing LMN signs in one region is enough to raise suspicion. If you see only upper motor neuron signs, other conditions like primary lateral sclerosis become more likely. If LMN and UMN signs are present in multiple regions, that strengthens the diagnosis under stricter criteria, but for suspected ALS, LMN signs in at least one region are the decisive finding.