**Core Concept**
Type I diabetes mellitus (DM) is an autoimmune disease characterized by the destruction of pancreatic beta cells, leading to absolute insulin deficiency. This results in hyperglycemia due to impaired glucose uptake by cells and excessive glucose production by the liver.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of type I DM. In this condition, the immune system mistakenly attacks and destroys the insulin-producing beta cells in the pancreas. This destruction is mediated by autoreactive T cells and autoantibodies against pancreatic antigens, such as glutamic acid decarboxylase (GAD) and insulin. The resulting absolute insulin deficiency leads to hyperglycemia, which is the hallmark of type I DM.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because type I DM is not caused by a lack of insulin receptors. Insulin receptors are present in type I DM, but the problem lies in the lack of insulin production due to beta-cell destruction.
**Option B:** This option is incorrect because type I DM is not associated with an excess of insulin. In fact, type I DM is characterized by absolute insulin deficiency, which leads to hyperglycemia.
**Option C:** This option is incorrect because type I DM is not caused by a genetic mutation in the insulin gene. While genetic factors may contribute to the risk of developing type I DM, the disease is primarily an autoimmune condition.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of type I DM is the presence of ketosis, which occurs when the body begins to break down fat for energy due to the lack of insulin. This can lead to diabetic ketoacidosis (DKA), a life-threatening complication of type I DM.

**Correct Answer:** A.