Final answer:
Brenda's symptoms that lack a known physical or genetic cause are referred to as an idiopathic disorder, which is a diagnosis based on symptoms rather than identifiable changes.
Step-by-step explanation:
When Brenda's doctor could not find any physical changes to explain her symptoms, the doctor refers to this as an idiopathic disorder. An idiopathic disorder is an illness without a known cause, in which the diagnosis is made based on symptoms rather than identifiable physical or genetic changes. This is consistent with the information provided that it is difficult to identify the cause of many nervous system disorders due to the lack of known associated genes, no obvious defects in brain structure, and the variability of symptom onset and display among patients.