In patients with clinically probable ALS, what other mimicking pathology do you rule out before establishing the diagnosis?  

By and large, I agree with the comments. The most important thing is not to give an incorrect diagnosis of ALS, with its dire prognosis, if there is a different cause for the patient's condition, such as MMN, neuropathy, or a spinal cord lesion. I am not in a hurry to give a diagnosis of ALS, in vi...

It is not a diagnosis of exclusion so you don't necessarily need to rule out other conditions before making the diagnosis if the patient meets the criteria. However, it is common practice to obtain/review imaging, especially of the spinal cord, to make sure some or all of the upper motor neuron sign...