## **Core Concept**
The diagnosis of diabetes mellitus is primarily based on the levels of blood glucose. The American Diabetes Association (ADA) provides guidelines for diagnosing diabetes through various tests, including fasting plasma glucose (FPG), oral glucose tolerance test (OGTT), and hemoglobin A1c (HbA1c). These criteria help in identifying individuals with diabetes to initiate appropriate management and prevent complications.

## **Why the Correct Answer is Right**
The ADA guidelines specify that a diagnosis of diabetes can be made if the fasting plasma glucose (FPG) is ≥126 mg/dL (7.0 mmol/L). This threshold is chosen because it corresponds to a level of glucose that is associated with an increased risk of diabetes-specific microvascular complications, such as diabetic retinopathy. The value of 126 mg/dL is derived from the distribution of fasting glucose levels in populations and the risk of developing diabetic complications.

## **Why Each Wrong Option is Incorrect**
- **Option A:** 200 mg/dL is a criterion for diagnosing diabetes using a random plasma glucose test (in the presence of symptoms) or a 2-hour plasma glucose value during an OGTT, not the fasting plasma glucose criterion.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the diagnosis of diabetes requires confirmation, unless the patient presents with classic symptoms of hyperglycemia (polyuria, polydipsia, and unexplained weight loss) and a random plasma glucose ≥200 mg/dL. For asymptomatic patients, a high result should be confirmed on a subsequent day.

## **Correct Answer:** . 126 mg/dL