Final answer:
Type 1 diabetes mellitus is primarily characterized by the destruction or dysfunction of the beta cells of the pancreas.
Step-by-step explanation:
Type 1 diabetes mellitus is primarily characterized by the destruction or dysfunction of the beta cells of the pancreas. In this condition, the immune system attacks and destroys the insulin-secreting beta cells, leading to a lack of insulin production. As a result, individuals with type 1 diabetes need to receive synthetic insulin through injection or infusion. Elevated free fatty acids in the plasma are not the primary pathological irregularity associated with this type of diabetes.