This is what I have personal experience with concerning doctors.

There are those who memorize to get thru school, and rely on memory to work.

Then there are those doctors who learn biological processes in the body and how they progress and lead to other things. Along with the memorization of anatomy etc, these doctors are more understanding of how the body really works.

Autoimmune issues typically are global. The antibodies circulate everywhere and attack tissues that have some code in the peptides that the antibodies recognize. So both sides of the body are typically at risk equally. Only if one side has some circulatory problem that prevents blood from getting there equally to the other, would it be one sided. Metabolic issues are the same. We don't see diabetics who get only one foot affected by neuropathy, as both sides usually are affected. They may not be equal perfectly because of some reduced blood flow perhaps.
My hypothyroidism affected both hands and feet equally. I was in my early 30's then. So my circulation was pretty normal.

Compressive issues in the spine, may however only affect one side. Previous trauma to a leg or arm, may predispose to one side. However, drug induced PN or toxins/poisons would be going all over the body too, and be more global.

You could ask that doctor to explain why autoimmune would not affect both sides... that may reveal something. He might be unable to answer that question, even.