## **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 timely 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 or higher. 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 FPG test requires an overnight fast of at least 8 hours.

## **Why Each Wrong Option is Incorrect**
- **Option A:** 100 mg/dL - This value corresponds to the impaired fasting glucose (pre-diabetes) category according to ADA guidelines, not diabetes.
- **Option B:** 110 mg/dL - Although this value indicates an elevated fasting glucose level, it is not the threshold for diagnosing diabetes according to the current ADA guidelines.
- **Option D:** 200 mg/dL - This value is relevant for the diagnosis of diabetes in the context of random plasma glucose levels (in the presence of symptoms of hyperglycemia) or after an OGTT, not specifically for fasting glucose.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that for the diagnosis of diabetes, the FPG value of 126 mg/dL or higher is used. Additionally, an HbA1c level of 6.5% or higher is also diagnostic of diabetes according to ADA guidelines. These criteria help in early detection and management of diabetes.

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