A 15-year-old boy is newly diagnosed with type 1 diabetes mellitus (T1DM). The nurse practitioner is educating both he and his mother about the disease. Which of the following statements is incorrect?

Choice A reason: T1DM is autoimmune in origin, where immune-mediated destruction of pancreatic beta cells leads to insulin deficiency. Choice B reason: T1DM often manifests in childhood or adolescence, consistent with onset prior to age 20. Choice C reason: Destruction of pancreatic beta cells results in lack of insulin production, explaining hyperglycemia in T1DM. Choice D reason: This is incorrect. First-line treatment for T1DM is insulin therapy, not metformin, which is used for type 2 diabetes.