**Core Concept**
Diabetes mellitus is a group of metabolic disorders characterized by chronic hyperglycemia (high blood sugar levels) due to defects in insulin secretion, insulin action, or both. It results from a combination of genetic and environmental factors that lead to impaired glucose metabolism.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of diabetes mellitus. Insulin, a hormone produced by the beta cells of the pancreas, plays a crucial role in regulating blood glucose levels. In diabetes mellitus, the body either does not produce enough insulin (type 1 diabetes) or is unable to effectively use the insulin it produces (type 2 diabetes). This leads to hyperglycemia, which can cause a range of complications if left untreated.

**Why Each Wrong Option is Incorrect**
* **Option A:** Insulin resistance is a key feature of type 2 diabetes, but it is not the defining characteristic of diabetes mellitus as a whole.
* **Option B:** Hypoglycemia is a potential complication of diabetes treatment, but it is not a characteristic of the disease itself.
* **Option C:** Hyperlipidemia can be a complication of diabetes, but it is not a defining feature of the disease.

**Clinical Pearl / High-Yield Fact**
The World Health Organization (WHO) defines diabetes mellitus as a fasting plasma glucose level of 126 mg/dL or higher, or a 2-hour plasma glucose level of 200 mg/dL or higher during an oral glucose tolerance test.

**Correct Answer:** D.